Tower tactics.

I once got lucky – I immediately bought Orrin (a specialist in ranged attacks), started developing him as my main hero, and over time, I replaced the gremlins with archers from the Castle, gargoyles with fanatics, and of course, I kept the mages and titans. And the knights too; the damage from the archers was quite impressive...
But in general, in my opinion, playing as Fortress is difficult, but if you're lucky with resources, it's a lot of fun.