The epic Battle of Alesia. Three rings of wall. One for the fort itself, one to invest the defenders, and one to defend the besiegers against the relief force. But this is AoE, and the game situation differs from the actual battle in a number of ways.
As an AoE scenario, this has to be rated as insanely hard. You have a huge base, with only minimal defenses relative to the size of the base. You are heavily outnumbered, so you have no hope of doing anything other than trying to hang on to what you have. You have no cavalry, and no realistic chance of getting any for a very long time, so you cannot sortie. All your enemies need to do is produce a bunch of catapults, and then the scenario is immediately and obviously pointless. In AoE, even the best of players can't do more than micromanage maybe two groups. Here, you will be under constant attack over 50% of the map, i.e. you will need to micromanage about 10 different situations continuously, otherwise your units do little more than commit suicide. In principle, you have plenty of resources, but a lot of them are right on the front lines, hence interdicted. You cannot get priests, so you will take enormous casualties.
The bottom line is that, one way or another, you are going to have to cheat in order to survive this scenario, and the amount of cheating required is actually quite significant. Accordingly, you may want to just enter the "home run" code right away and move on.
If you are an AoE expert, and you want to try to beat a scenario that is virtually unplayable, there are several things that you can try. First, if a micro game is impossible, try playing a macro game. Get 20 or so villagers, get a massive food-wood economy, pump out the chariots, and take it from there. However, you are still outnumbered three to one. Second, abandon your position entirely and set up somewhere defensible. However, you would need to analyze the whole map and some of the play first to figure out where that might be, if in fact such a position exists. Third, you can assume that the AI will actually play quite badly, and use any scenario bugs to your advantage. In fact, such bugs do exist. Finally, you could incorporate elements of all three approaches.
The next thing to recognize is that Yellow does not use missile troops. You should concentrate your archers against Yellow while you build up. Red may avoid using missile troops as well.
The next thing to recognize is that you will need cavalry. Caesar cannot be used in combat, except in extreme emergencies, and because you don't know how bad things will get in the future, this effectively means that you can never use him. If you don't sortie to wipe out attacking catapults, then they can blow away your walls safely and let the rest of the troops overrun you. Even if the catapults take on your Towers directly, which at the beginning, they can do safely, at a minimum you would need to upgrade their range in order to have a chance of fighting them off. This would require a Market (127 wood) and Craftsmanship (240 food, 200 wood). Since you are short 40 food, you would need to get at least a farm (63 wood), and now you are also short 190 wood. In any case, this only gives you a chance of fighting off one basic catapult, and no chance at all against anything more. On the other hand, getting a couple of Chariots will cost 127 wood, plus 80 food and 120 wood, which only leaves you short by 47 wood, and leaves you 120 food for other purposes.
The next thing to recognize is that a lot of your villagers will need to be standing around doing nothing, ready to repair walls and Towers. Your base is too big to avoid this, because you have no way of predicting where the attacks will be. This will significantly hurt your economy.
The next thing to recognize is that collecting ore could be problematic. On the one hand, you absolutely must get your food-wood economy ramped up as high as possible right away, and you just won't have anyone available to collect gold. On the other hand, all of your ore deposits might be interdicted. The stone deposit is well guarded, so it is relatively safe, and you will need stone for repairs and construction. One of your gold deposits can actually be exploited safely, but it would require a cheat to say how, so we will assume that gold is unavailable until further notice. This means that you will have to fight with Chariots, Scouts, Bowmen, Slingers, and Towers. A fine Roman Order of Battle. You do have your original archers as well. We won't mention Axemen here.
The next thing to recognize is that your position is too narrow to be tenable. As much as possible, you need to extend your outer wall to the edge of the map. Effectively, you need to have a separate wall around each of Red (the inner wall), Orange (the eastern wall), and Yellow (the western wall). Not only does this give you more space to work with, and safe space, but it actually shortens the length of wall that you need to defend, and it allows you to concentrate certain types of troops against each of your enemies, because each of your enemies uses a specific force composition, and in isolation, they all have weaknesses.
The next thing to recognize is that you will not be able to properly manage all of your forces. For example, just building your walls will take 100% of your concentration. In the meantime, your workers are all standing around idle, and nobody is monitoring the battles. Doing all the required actions in real time just isn't going to work. Take a few minutes to have a really good look around your base, and at least figure out where you definitely want to build walls and collect resources. Then restart the scenario and actually do these things. After that, learn how to use the F3 function. This is not a cheat or a bug, but it is not how AoE is intended to be played.
The first thing that we are going to do is grab some more space. The unexplored south map corner looks large enough to give us a lot of building room, but too small to contain an enemy base. Send Caesar, who happens to be an excellent scout, over there right away.
The second thing that needs to happen right away is that our walls need to be completed. The inner wall is continuous, but the outer wall has a large gap in the west, and a smaller gap in the east. Fortunately, we already have combat engineers in both areas. However, in the east, we will leave the gap, and instead connect the wall to the map edge.
If we need to cross our walls for any reason, we will simply delete a section and rebuild it later. We will do this until the situation has stabilized, and we can have permanent "gates". We certainly have enough stone that we can waste some in this way.
Put your initial villagers in the west to work building stonework. Two can build a Stable in the east and then collect wood. The remaining 2 can follow Caesar around, building walls as appropriate. Produce 2 more, and put them on wood. As soon as you have enough for a Granary, build one near your forage area and start bringing in food. This operation might be interdicted, in which case you will need to switch to farming in a safe area.
By the 5 minute mark, you should have produced 3 Chariots and 3 new villagers. Your outer wall should be complete. In the west, it bulges out into a thin forest. This is basically necessary, due to the river that runs into your base, and the presence of shallows. You will need to pay attention to this area, because Yellow may try to cut down the forest. Your outer wall reaches to the SW and SE map edges. You should have explored almost half the map. You should have built one new Tower in the SW. Four villagers are standing around in the west, one in the east. Two are collecting food, two wood. You should have killed 5 or so enemy units already and might be feeling pretty good. And you only used one cheat so far - that you would definitely need Chariots in the east, but nowhere else.
Save the game and think about what must happen next.
There are now three immediate objectives: first, to pump the wood-food economy as high as possible, second, to develop the stone economy, and third, to seize control of the north part of the map, thereby separating Yellow and Orange.
Administratively, it is time to figure out where your archers are and to reposition them if necessary. In general, they should be in safe-ish areas, not right against the walls. You should assign a single control group to all your archers. This will hardly help you to move them, but it will allow you to locate them all quickly. It will also allow you to give them all "stand fast" orders simultaneously, and that is useful for archers.
You also want to keep track of where your villagers are, especially when you have all 20. With half of them standing around waiting for something bad to happen, you really need to be able to locate them all quickly. The recommended procedure is to make notes on paper.
Fighting off the hordes
Shortly after the 5 minute mark, right on schedule, the first Orange catapult shows up. Immediately delete a wall section and hunt it down with your chariots. If you had not prepared to deal with it, this one unit alone would do significant damage. Your chariots will take damage from other Orange units, so retreat them as soon as the catapult is dead. One might ask whether it is better to take damage on an unrepairable unit that costs 100 points of common resources or on a repairable unit that costs 75 points of a limited resource. In the long run, it is clear that the Chariot is more expendable, but at present, all resources are limited. However, the real point is that certain units, such as catapults, must be destroyed immediately, by any means necessary, because they do so much damage, and perhaps with impunity.
You will now be facing nearly continuous attacks by your three main enemies. Yellow uses Cavalry and Barracks infantry. Red does the same. Orange uses archers and catapults, but will prefer horse archers and Ballistae after entering the Iron Age. Red and Yellow seem to remain in the Bronze Age, perhaps because they are spending more food on military units. Move your archers around as appropriate, but don't spend too much time on this. Keep your walls in good shape, but you won't be able to keep them in perfect shape, primarily because you will simply be too busy elsewhere.
Once Orange starts attacking with significant numbers of Iron Age units, you have to build more Towers in the primary attack area. Use two villagers to build each Tower, wait until the coast is clear, and definitely don't order the Tower until your villagers are standing right next to it.
Upgrade the defenses wherever your enemies prefer to attack. Interdict the gold deposit that you can see inside Alesia. This is not nice, but as we say in Rome, "Quid pro quo". After you have 20 villagers, get the most important upgrades in priority sequence. Produce a Scout and send him over to the north. Delete a wall section and start exploring. Extend your wall to the NE map edge.
At some point you will realize that there is not enough wood inside your initial base. There would be enough if you could heal your units, but you can't. You will need to distance-harvest a lot of straggler trees. There may be enough wood inside your expanded base, but you will still need to be careful with your resources. There is quite a lot of food available via hunting, but hunting requires too much micromanagement for it to be feasible in this scenario.
Save the game at the 20 minute mark.
At this time, you should still have your original 4 Legions and 13 archers, plus Caesar. You should also have 20 villagers, a Scout, and 7-10 Chariots. The Chariots should now be distributed in 3 or 4 groups, so that you can quickly respond to siege weapons attacking from any direction. You should have +3 missile weapon range and level 3 walls. You should have sealed off Orange on both sides, or close to it. You should have killed about 100 units for no loss.
Your thumbs should be prickling, because something wicked is about to arrive. Watch the mini-map very closely, and check out any new dots instantly.
A few minutes after the Brown Catapults attack, Red and Yellow start attacking with catapults and priests. You will no longer be able to control the situation. You will now be losing units and Towers, but hopefully not in large numbers. In some sense, the game has now become normal - Situation Normal, All Fouled Up. Your enemies are now throwing everything they can at you, and they haven't got any surprises left.
Attacking into the teeth of the dragon
As soon as you open a hole in your wall to hunt down the first Orange catapult, their villagers start trying to run through it to scout your territory. Even your initial Towers don't permit it, and the north is blocked to them as well. However, you will be blocking them absolutely in the north with a wall, and you will be strongly upgrading your defenses near your opening, because this is also where they like to attack you. It takes a long time to kill these villagers with missile fire, because they run very fast, and they dodge very effectively. However, you also have Chariots available, and they will kill a villager in two blows, and they are fast enough to hunt them down, and you can't dodge melee attacks. This means that Orange will experience a serious drain on their economy.
None of the Orange units outrange your Towers. Even though many Orange attacks can do serious damage to your Towers, they will always be under automatic return fire. Up until the time that your other enemies start attacking you with catapults, which happens at about the 25 minute mark, you should be able to kill the Orange catapults quickly with your Chariots, and to keep your Towers in good repair. After that, because of micromanagement overload, you will no longer be able to avoid losing Towers and villagers. However, Orange is burning through an awful lot of resources. In conjunction with the loss of their scouting villagers, this means that, somewhere around the 40 minute mark, the Orange population will actually be in serious decline. This means that you will be able to push a Scout into their territory and get some sense of where things are. You will then be able to hurl your chariots at Orange and break them completely. Don't fight the Towers; go for the mobile units, especially the villagers. Don't worry about losing your chariots; kill the Orange units.
When Orange is out of units, and maybe a bit earlier, you realize that you must have some catapults of your own. The best place to get gold is in the NW corner of your base. Put the Storage Pit right against the outside wall; this is the safest place for it. It turns out that no one bothers you there.
Red only has access to 11 gold mines. Red has access to a massive amount of stone, but doesn't use it for much. Red also has only limited amounts of food and wood, but their lack of gold is what will kill them. Assuming an average cost of 40 gold per unit, Red can only produce about 110 units. Sure enough, once you have killed about 110 of their units, Red stops attacking you. At that point, you could go into Alesia with just one Catapult and kill Vercingetorix, or you could kill Yellow first.
If you pushed your western wall far enough forward, then Yellow will build some buildings close enough to you that you can bring them under fire from inside your base. This will give you the opportunity to shoot up their repairmen. Once you get catapults, you can really do a job on them. Because the AI will send repairmen out to die without consideration of the ability to replace them, you will be able to kill Yellow off by killing all their villagers. Yellow will still have some military units left at this time, and gold on the ground. They also have access to significant amounts of stone, but they are not interested. At this point, you will be able to finish Yellow off with just one Catapult.