You are relatively safe behind your walls. The enemy is attacking mainly your castle, sending small attack parties over the bridge in the middle of the map. The castle can easily cope with the camels and horses, however to counter siege weapons (after a short while, they will start sending trebuchets) you should keep a few knights inside the castle or nearby.
Start by leading the sheep from outside the walls towards your town centers and invest heavily in villagers, so you can gather ressources and advance technologically asap. Don't put too many villagers on stone, you won't need much.
Meanwhile, send your army from the southeast towards the west along the river (for sentimental reasons, you might be inclined to save Karl Martell by sending him to your town, but it doesn't really matter. If you do so, better use the ford near the north-east edge of the map, not following the road, as it will lead you into some enemies) until you reach the yellow enemy village, here destroy the town center and kill as many villagers as you can get hold on (some might be fishing a bit towards the north, a few more are in the south at the gold or cutting wood). Soon a bunch of camel riders and other cavalry will counterattack you, here you will have to sacrifice your axe throwers, but save all your knights by retreating into your castle (you cannot win against the camels). When the castle has killed all enemy riders, send a villager to repair it (might be down to half its hitpoints) and send your knights back toward the yellow village in order to raze stables and kill more villagers (all other buildings are low priority, even the barracks), until a new counterattack party is threatening you and you have to retreat again. Do this several times and the yellow enemy will give up soon, that effectively halves the number of enemies you have to cope with from now on. When done properly, you will have lost none of your 15 knights.
The green enemy has walls around his town, but you can stand his attacks relatively easy until you have developed cannons, that is the easiest and least dangerous way. Raze it completely and you are done.
The market carts that are mentioned in the objectives are obviously just a distraction. You will find them inside the green enemy's town, but when you are so far to get hold of them, the town is toast anyway. You can bring them out to use them for a trade route (you are unable to build trade carts in the market in this scenario), but that is useless at that point, as you are just a few seconds away from a win.