Combat is straightforward right now, and that's not a terrible thing.
However, there is room to expand on this and make it more user interactive.
Idea 1: Stage based combat
A series of 3 to 5 user interactions pulled from a list of scenarios that allow the player to make decisions that may turn the odds of the battle in their favor. The defenses of the fort could come into play, and the attack rank of the player could also be used to really set up some neat interactions. Some scenarios might lead to users retreating and saving more units than they normally would have, where bad decisions might lead to getting flattened by enemy defenses. It would be a fun place to expand on and tinker around with.
Combat is straightforward right now, and that's not a terrible thing.
However, there is room to expand on this and make it more user interactive.
Idea 1: Stage based combat