Information posted by Riddick via message:
If anyone is occupied and you have time on your hands to annoy your occupier. Build one slinger and hit rebel. This forces the occupying army back outside your wall to recapture your city. If your wall is a high enough level each time you do this the attacker will lose a number of front line units, usually around 4 steam giants, compared to your one slinger. Then you build another slinger and do it again and again, until you either get bored or the attacker moves on with a depleted army depending how soon he comes back online.