I think it would be cool if there was a special unit that, after killing an enemy, takes the enemies uniform and sneaks into enemy cities and, lets say, starts a riot or steals resources. When a riot starts, the victim player's productivity in gathering resources is obstructed for a set amount of time (a riot icon appears on screen). If a unit is stealing resources, the unit comes out of his disguise but runs at an increased speed. Edit: These units could have a cooldown time so players wouldn't spam them. To counter this, players could get a gate upgrade that puts guards at the gate (guards could have a 60% chance of spotting a espionage unit) And every time a guard catches an espionage unit the chance of doing it again increases. Another upgrade that would be cool is to have is archers on the wall. You wouldn't be able to control the archers, but they would show on the wall and attack enemies + increase sight of the walls.