This is probably why D&D castles all have walls that are thicker than 5 feet.
Incorporeal creatures are immune to critical hits... They move in any direction (including up or down) at will. They do not need to walk on the ground. They can pass through solid objects at will, although they cannot see when their eyes are within solid matter.
The incorporeal subtype specifies that they can't go though an object that is wider than the creature's space. But a castle wall is nothing. They just 'fly' over it. Or, if they want to be sneaky, they just go into the wall, then climb it like a ladder, cross over the top, and descend the other side, being inside the wall (and thus not visible) the entire time.
So, what you're asking for is the whole city to be built of bunkers.I could see a few buildings being setup like that after the first time this tactic is used, but not a whole city.
Incorporeal creatures can, however, be affected normallyby the natural attacks of other incorporeal creatures. Anincorporeal creature cannot occupy the same space as anotherincorporeal creature.
Saw this todayLibris Mortis, p141QuoteIncorporeal creatures can, however, be affected normallyby the natural attacks of other incorporeal creatures. Anincorporeal creature cannot occupy the same space as anotherincorporeal creature.You said the box was large. You probably didn't realize how large Though as pointed out elsewhere, they could just fly over the wall themselves. Or move just underground up to the wall, up the inside surface of the wall, across the top (still inside the wall), down the inside, then through the ground -- all the while staying out of sight and undetectable.