The tower is bringing to mind a general discussion about mob spawn rates.
And i am starting to suspect that ANet should revise the mechanics of such.
What i am observing suggests that any mob of a specific kind (court, krait and so on) can spawn on any point designated to their kind.
End result is that you can face a spawn wave simply because you cleared a few spots around you. Right before the mobs killed by, say, a earlier zerg rush in a different part of the map, came due for a respawn.
This can then give the impression that mobs continue to spawn and spawn without respite.