Moving to contact
Move Guy Josselyn down the road until he contacts the French army.
Organize the units. They should mostly have "stand fast" orders.
You have two trebuchets. These are your best scouts, because they are not generally vulnerable to monks. As you move them forward (towards the center of the map), you make contact with the French artillery. At this point, you will have a population of 65, you will be missing certain key units, and you will be low on workers. There is a completely unreasonable way to perhaps circumvent this problem, which is described below. Of course, you could always disband some units to make room in the roster later. But the main problem at this point is that you have no way of building anything, and no way of restoring any damage. If you had even 100 wood, this problem could eventually be solved, but you don't.
Make sure that the artillery does not take any action without your express orders.
Continue to move the trebuchets forward. Almost immediately, you make contact with Burgundy in the center of the map. This is their main military base, and will remain so unless you ignore the other base.
With no reasonable option available, you need to put in a frontal assault against the Burgundian base. But even in this context, you should be using Raid and Ambush tactics. Do not unpack the trebuchets except in special situations. Your main assault force is the six Bombard Cannons, but this needs to be a very delicate assault.
Each time you attack a new Tower, a relatively small group of defenders comes out to play. Of course, your Bombard Cannons need to withdraw at a moment's notice, while your other units deal with the incoming force. With properly executed Ambush tactics, you should take very little damage, but it is unlikely to be zero. So, Tower by Tower, you reduce the Burgundian base.
In the meantime, you should be scouting to the west south of the river. Since the trebuchets are otherwise occupied, you have to use light cavalry, so be prepared for monks. In the SW corner of the map, you find another Burgundian base. This is their main production base, according to scenario design. By the time you get there, there will be six villagers and two fishing boats in this area. Burgundy does not seem to be able to build any more of them. You need to take out this base ASAP. The main reason is that they are consuming resources that would be better used for the glory of France, but also, if you ignore this base, they will do their best to harass you later.
The only thing reasonably available to deal with the SW Burgundy base is the heavy cavalry, because the bulk of your army is engaged in heavy combat, and the SW corner is a long way away. As it turns out, all you have to do is kill six villagers and one fishing boat, but you have no reasonable way of knowing this a priori. Incidentally, killing a fishing boat is definitely not a trivial proposition with the force you have available. You should be able to catch most of the villagers away from the Town Center, and this is the first priority. After that, destroy the monastery, the two barracks, the Town Center, the farmers, and the dock, in that order, and then run away. Burgundy responds to attacks in this area with their entire mobile force (what's left of it). After all, their survival depends on this base. Your scouts in the countryside should be able to catch some of this force on the march.
By the time the SW Burgundian base has been neutralized, there won't be much left of their central base. There is a flag next to the central Burgundian Town Center, so you might have some idea that something special might happen soon, but you don't know exactly what. Should you or should you not destroy a particular building? After all, that building might come over to your side. Since the location of the TC is not favorable anyway, you destroy it. It turns out that that is the trigger. As soon as the central Burgundy TC is destroyed, they give you 150 stone, 50 gold, 400 food, and at least enough wood to build several buildings.
Completely unreasonable attack on Burgundy
There is a way to build up your force to population 75 before collecting the French artillery. Then you would have 25 spots in the roster for monks, cavalry archers, ships, and more workers. However, it requires clairvoyance and prescience, and is a completely unreasonable way to play the game.
After activating the main French army, you can sneak a few units along the south edge of the map without activating the French artillery. However, the trebuchets, light cavalry, and even the archers cannot do it. (Actually, you could use the villagers to cut a path through the trees along the very south map edge, using the Grab-and-Dump method, but not only is this pretty ridiculous, it will take too long.) Practically, the only units available are the 6 + 2 heavy cavalry. So you are heading off into the wild black yonder without any scouting at all. In the SW map corner, you just happen to know that you have to kill six villagers (some of which may still be arriving) and one fishing boat, and maybe the Town Center as well, because the last villager hides in there, no more, and no less. As soon as you kill these units, Burgundy surrenders. Just about everything they have left commits suicide. When their central Town Center blows up, you get a bunch of wood and other resources. This will allow you to build up.
But you still will need the food and stone in the middle of the map, so after all this, your cavalry and villagers have to walk a narrowly defined path to the middle of the map, avoiding the British, all without scouting, and relying on the fact that the Burgundian wall is not complete. Then you need to build defenses in two areas, because your archers, light cavalry, and trebuchets cannot advance without activating the French artillery. (This means that one villager needed to stay behind.) You will need to defend the middle base for quite some time against fairly serious attacks with only a handful of cavalry as a mobile force, having no artillery or even missile troops there.
There are three ways to join your two groups. One is to build up to population 75, after which you can activate the artillery and move freely with all your units. The second is to use villagers to cut through the trees near the east side of the map, creating a path far enough away from the artillery. However, the second method takes as long as the first, so there really is no point to it. The third way is to cut through the forest with your trebuchets, but this is still fairly slow.
Once you've activated the French artillery, the game becomes normal, except that you are at population 90.
Immediately wall your army in in the ex-Burgundian base and start production operations. Ignore the S corner and the SE map area for now; it seems strategically unsound to build there at this point in time.
As soon as you have a Town Center, your infantry can be healed. As soon as you have a castle, your cavalry can be healed. Siege weapons can be repaired with resources and a villager. It may be some time before you have monks, because your base is too small to put a lot of houses there right now. One castle is a priority; Towers are not advisable. Many upgrades won't apply to your units - keep this in mind when building.
The situation might be relatively calm for a while, but soon the countryside will be crawling with enemy units. As soon as reasonably possible, start reclaiming the land. Do this the way the Dutch build dikes: build a wall, then build another wall beyond that, and then another one. You need to use the Maze Complex approach. The enemy will use siege weapons in large numbers, and a simple complete wall just won't stand up.
The red British don't seem to do much. All the action is by Shrewsbury's army. They attack with a flood of Cavaliers and up to twenty battering rams at a time, and they keep this up for quite some time. Against an attack force like this, Towers are a waste of stone, castles are almost useless, and simple walls don't work. You need big walls, thick walls, multiple wall lines, guided pathways to nowhere, new walls being created as the attack goes in, artillery hammering the rams from inside the wall, cavalry hammering the rams outside the wall, machine guns hammering the escorts, in fact a full spectrum defense. Catching the rams on the march is very important. You can provoke this situation by building your walls in a way that suggests that there is a relatively easy or even open way in to your town over there somewhere, as long as the attacker walks all the way around the main defenses. Palisade Walls are actually very important, because they are essentially free, can be removed and replaced at a moment's notice, stand up remarkably well against non-siege weapon attacks, and have a serious channeling effect. This is all part of the Maze Complex Defense.
The castle is a key element of the defense. At some point, one of your defenders, probably wounded, perhaps seriously wounded, is running away from certain death, having diverted some of the attacking units. Running through a gate doesn't work if the pursuing units are too close behind. Running into a boat also works, but running into a castle works best, because not only is the running unit immediately safe, but the pursuers get hit in the face with a storm of arrows and maybe some other nasty surprises. This situation will happen again and again if you're playing at the limit, and it is only one of the reasons why a castle is so important.
Eventually your defense will be strong enough that you can start thinking of attacking. Shrewsbury is the logical target for two reasons. First, the red British don't seem to be doing much, so maybe you can ignore them for now. Second, scouting suggests that Shrewsbury can be isolated and contained. You later find out that this is indeed the case.
You attack Shrewsbury by investing his position with walls. After each of his main attack waves, you rush forward and clear out the remnants in the field, and then very quickly advance your Wall Complex. You may not be able to build a complete layer before the next attack, and the next attack may well destroy part of your new wall, but as long as your Wall Complex advances at all, you are making progress. Creative use of deception, diversion, and Palisade Walls are very important here.
Eventually your Wall Complex is close enough to Shrewsbury's base that, on your next push, you can seal one of their two gates with stone. Do so. Repeat with the other gate when possible. This completes the investment, because there are only two exits from their base. Shrewsbury's army is now helpless, because they have no effective static defenses and won't build any, and a cavalry/ram force, which is all they ever produce, will not be able to break through the sealed gates. Your next step is to bring up your own siege weapons, which DO fire at range, and lay waste to as much of the base as you can reach. In principle, you could now wait for them to starve, but in practice, you don't know how long that will take (and, anyway, they may get tribute), so you do have to complete the job. If you have got this far, it is relatively easy to unseal a gate and kill everything inside the base that moves or could produce something that moves. You might consider leaving something alive for later conversion, but they just don't stop building, so anything that moves or can build has to go.
Only the British based at Castillon remain. A river divides their territory from yours, and there is only one ford. Seize the ford, and don't let anyone through who doesn't speak French. Explore as much of the British territory as you can. Outside of Castillon, which is large and fortified, there are two outposts based on castles and a few isolated Towers. There are a few scouts wandering around, but no field army. As has already been mentioned, the red British seem to be purely defensive. This is good for you in some ways, but it means that they will have hordes of units available when you finally do attack them. At this point you get secure access to several substantial gold deposits, and one stone deposit, as well as all the farms that Burgundy built for you. You won't need those resources, but your villagers might as well do something.
The red British produce hordes of Elite Longbowmen and some heavy cavalry. Longbowmen are very powerful units, but they do have certain weaknesses. First of all, in the game, they are pwned by super-heavy cavalry under any circumstances. They are also quite weak against trebuchets, which have enormous piercing armor. This means that they are ineffective as an anti-trebuchet reaction force, and you will take advantage. What you will do, literally for the rest of the scenario, is move your trebuchets forward, attack something, and keep shooting until the Longbowmen arrive. Then you can pack up your trebuchets in good order, retreat them, and repair them, while your Paladins slaughter another bunch of Longbowmen. You do need to be a bit careful, because the British do use some heavy cavalry, but your Paladins will smash them in the open field as well. The red British do have a couple of trebuchets, but you can wipe them out with your Bombard Cannons or Paladins, as appropriate.
While these attack-retreat-repair cycles go on, the British keep scouting. Since AI scouts don't fight back, you can take over a bunch of the Longbowmen with your monks with complete impunity, and you should do so. There's no real point to converting the British heavy cavalry.
Take out the isolated Towers first, then the larger outposts, and then go for Castillon itself. It is easier to grind the city up from the north first. Keep adding new wall compartments as you advance. Keep an eye on the enemy villagers. They are likely to be collecting wood outside their base by now, so you can kill them in heaps and generally deny them access to the wood.
Castillon has only two gates. Once you have sealed them both off, the red British are completely helpless.