maybe a little tower with npc spawners?

What about a area with water and a bridge connecting the two sides forcing npcs to take the risky route

Maybe you should use the big flat grass area as somekind of trench defense area where theres a tunnel leading into the trench under the battlefield and on the grass land above the tunnel there would be like some cover for the S/NPCs also that flat concrete area could be a tower area for a large and tall tower then that tower could connect to a sky island above the map and the grass area near the 2 towers could be a tunnel area system. And if you do add that sky island idea. you can make it kinda look like heaven with a gate and a temple and that tunnel system could connect to a area below the map with a underworld/hell thing where its a labyrinth and one big room in the mid with a hole leading into the void or some kind of grinder.