There seems to be no limitation on monster spawn location during the waves by the beacons.
The biggest animals spawn in the tiniest rooms, sometimes right on top of the core even.
The reprecussion is, the more you build, the more obstructed are the turrets, walls are no defence at all if monsters can spawn on either side if it. Especially if its below ceilings and turrets keep on firing against the walls. Hope there is a way to limit spawns to the outer rims of the colony shield.