So the basic premise is tower defense meets RTS. Instead of towers, you command guerrillas. You'd place different types of guerrilla soldiers and traps along enemy routes or specialized units that would fight with enemies, steal supplies, take captives. But you'd had to consider deployment, reinforcements, and maintenance so you'd also have to place camps and other base type structures to allow your guerrillas to resupply during "waves" and recover between them. Things like elevation, concealment, cover, distance between camps and your guerrillas, etc would be important
It doesn't matter if it has been done already, as long as you do it better. Medal of Honor was doing well, then Call of Duty came along and crushed it. Alone in the Dark had been done, then Resident Evil came along and crushed it. Doom had been done, then Halo came along and crushed it...
