Spartacus. What a great movie that was! But then, any movie with Kirk Douglas was great.
Spartacus. What a strange scenario! Right away, you can predict that it will be a slaughter. Check the Achievements menu and you will see that Spartacus has a large army, but no villagers. Therefore, any unit they lose is permanently gone. You, on the other hand, have a good economy, with plenty of gold, stone, and food. You haven't really found any wood yet, but this is AoE, and wood is never in short supply. You have a well-developed base already, so it's really just a matter of pumping out an army and hunting down the rebels. So the seers say ...
But it's all hogwash. The seers lie. Your enemy has priests, and they can convert friendly villagers, and they can quickly build up a huge economy. This is almost certainly going to happen, but random factors may prevent it, which has a huge impact on the outcome of the scenario. Keep checking the Achievements to see how many conversions have happened, and to get other related information. If the priests are extremely aggressive and lucky, you probably need to just restart the scenario, because you won't have a chance. Two conversions by the time you get Writing is fairly normal.
The enemy troops are all, with a few exceptions, far superior to anything you can produce at present, and will be until far in the future. All they need to do is send a group of Heavy Horse Archers into your base, and you will die. Period. If this happens too early, just restart the scenario, because it's pointless to continue. Because you simply cannot face the enemy army in the open, your only reasonable option is to build massive defenses, based on a nearly complete wall-in and fully upgraded Iron Age Towers, before you do anything at all to attract attention. Woe betide you if an enemy villager should happen to run into your base too early!
However, it's also pointless to build Towers except in certain specialized circumstances, because all the enemy ranged units outrange your Towers until the Iron Age, and even then, all their siege equipment outranges your Towers. Therefore you won't build Towers at all, except for a Tower Maze at the opening to your base. Note that Romans have super-cheap Towers.
In any case, what you need to do is spend your initial food on one Scout, Writing, and Wheel. Collect primarily food, but also gold and stone, with your villagers. Produce 10 more villagers as food comes in. Even 25 villagers will not be extreme in this scenario. You will need to disband some of them later, but right now, you need a flood of resources coming in. It won't be long before wood becomes far more important than food.
Explore your corner first and then up to the near edge of Italian territory. Seal off all gaps between the forests with walls, leaving only one opening. You will be hard pressed to defend even that one. Make sure that you enclose a lot of resources. In fact, your wall needs to extend all the way to the river, where you build a dock.
As soon as your dock is finished, start pumping out War Galleys, and flood the map with them. The Rebels may decide to have none of this, and might attack one of your plain walls, or maybe the front door. If they seriously want to smash through your wall, you can't stop them. What you can do is convert some of the units that run into your base, and fight the rest of the attackers with those. You did produce a lot of priests, right? After that, there will be a short, but savage series of assaults right into your base. Put your Cavalry against their siege weapons, chariots against their priests, and priests against the toughest units. You will lose a lot of units, but the action is right in your base, and you have a huge economy, so you can replace any casualties easily, except that in practice, you will have too many converted units to produce a lot of new ones.
Note that if two priests target one enemy unit, that unit goes catatonic and can't fight back. This is not an efficient way of converting units, but it is safer for the priests, assuming that the target doesn't have a lot of friends along.
Pretty soon, the Slaves run out of units, and then you realize that, despite having villagers and income, they do not seem to be able to produce any new units. So the seers were right, after all!
All you need is priests
This leads to an even better strategy. All you are going to do is produce a bunch of priests, maybe 20 or so, enough Cavalry to engage the enemy siege weapons, maybe 5 or so, and some chariots to hunt down the enemy priests. You don't need to bother with researching Iron Age or any later technologies, so you won't need a lot of resources other than gold. Therefore you won't need to enclose a lot of resources in your base.
Start the scenario the same way as in the previous strategy, except this time you stop at 20 villagers. To recap, spend your initial food on one Scout, Writing, and Wheel. Collect primarily food, but also gold and stone, with your villagers. Produce 10 more villagers as food comes in. You could disband some of them later, but right now, you need a flood of resources coming in. It won't be long before gold becomes far more important than food.
Explore your corner first and then up to the near edge of Italian territory. Set up a wall to enclose one gold/stone deposit to the N of your initial position, and one to the E. This will give you way more resources than you will need for this short but savage fight. Your wall will enclose somewhat less than 20% of the map, and will have only one opening, where you will build a Tower Maze. Don't build Towers at other locations, because you will have no way to protect them against enemy siege weapons.
As soon as you have 5 (or even 3) Cavalry, go hunting. The only enemy units that you can actually beat at this time are their chariots, and even then, you need at least two-to-one odds. The Rebels are not happy that you are beating on their light cavalry, so they charge your base with a bunch of Heavy Horse Archers. They outrange your Towers at present, so they think that they're on to a good thing, but while your Cavalry has been roaming around, your Temple was pumping out priests. Now, generally a Heavy Horse Archer will beat a priest, but when you double-team them, they can't fight back. If the Rebels let you pick off their horse archers one by one, well and good, but they may well rush into your base now. This gives your Cavalry and Towers a crack at them as well, but you need to be careful. In general, do not shoot at units that you are trying to convert, and throw the converted units into combat immediately. If you can manage it, send weak and wounded units to safety, but the primary task here is to possess as many of the horse archers as you can. You want at least three, because a group of at least three Heavy Horse Archers will be able to wipe out the entire Rebel force by itself, given enough time and healing. When there are no more enemy horse archers alive, regroup, heal, and repair.
Although you were aiming for 20 priests, in practice, you won't have more than 12, because you can't control more anyway, and the enemy doesn't attack in such huge groups that you need more. So this means that you won't produce more than 32 units beyond your initial 10 villagers. Everything else in your army will be converted.
When you are ready to move out again, go hunt some more, but this time, use the horse archers. From time to time, the Rebels will attack your base with a group of heavy units. It doesn't really matter how strong your wall is; they will cut through it in seconds. However, as soon as they run into your priests, some of them get converted, and the rest die. Since the Rebels don't appear to be allowed to produce anything, they run out of units pretty quick. The Italians may even have some villagers left when the dust settles.