I ran into this problem when I was introducing Annwn, but the solution I came up with probably works on other worlds as well. The one major city we see on Annwn in its intro has walls of Adamant (I think maybe silver given the Adamant enchantment, IIRC) but an open top. The solution was just simply "wards." IE various kinds of magic (such as force fields) kept flying things from going over the wall and invading the city. Of course wards can be attacked, there's a scene in book two where a place's wards get a hole punctured into them by a specialized magical artifact called a wardbuster, so it's not a guarantee. The wardbusters make a lot of noise and vibration though, buying time for the defenders to rally to fight the invaders.

I do like your pyramid idea, though. I may use that somewhere.