Praetorians - HD Remaster

First play on easy.

Now take all your cavalry, legionaries, spearmen, gold wagons, and only one of physician, infantry, archer then go to the very top corner outside your fortress. Follow the path along the top below the ridge next to the black emptiness. Keep going until you can go down into the town and move the gold wagons by it to recruit a lot more cavalry.

While your moving this army make only archers at the barracks. Move all your archers and slingers onto the wall by the two gate. The far top portion of the wall won't be getting attacked past the outcropping with a tower. Move your commander to cover the most units in the fort and move the physician around as needed. Now the one infantry left in your castle can be used to repair the gates, as long as no battering rams attack it you can out repair the damage.

Once you finally get your mercenaries with the gold move them down along a path on the ridge to the beach. By the time they get close your reinforcements should be spawning in. Follow along and try to protect them, they go straight through the enemies main force so be careful. As far as I'm aware you only need to get their commander to the fort. The reinforcements take some detours to kill enemy siege so at this point you may need to make some legionaries or spearmen instead of archers to help out if your army out of the fort is getting thin. When the commander makes it to the fort your mission should end.