just look at arcade survival shooters online and tower defense where you put turrets along a path. Your game is those two genres combined.
Levels i mean basic variation on the empty room. maybe four pillars, maybe a small dividing wall in the middle, maybe an area of lots of little rocks, any little variety which adds a bit more strategy. That's the tiles i added to still not dead, it just adds a little more thought about how to approach the fight, and incentives to use different weapon types, movements or tactics.