Sounds like Horde mode to me. I'm fine with that but would like to see more details. The way you're putting it sounds absolutely chaotic and a possible lag fest. I would do it as an instance from the tower's location with an 8 man team MAX. You get placed in relatively large area (similar to patrol maps) and enemies will spawn from a few different locations on one side of the map through the wall/buildings. On top of that, they will spawn drop ships in the middle of the map. Waves of say 20-40 enemies but instead of clearing waves to start the next one, each wave is on a timer, so enemies just accumulate if you don't kill them. Maybe have 9 waves where every third has some mini boss. Defeat comes in the form of enemies (like VIPs) maybe escaping through portals into the city or destroying an object. Guardians can respawn at the end of each wave but can be revived--kind of depends on rules of defeat. As for drop ships being able to spawn where ever. Just say there's a machine or some power that prevents them from using portal devices into the city and can only get to that point. I would also make it so spawn rates, enemy types, and mini boss abilities are randomized to some degree so no two versions of the game are the same. Lastly, have a tracker at the Tower that shows progress of defense on all servers.