Can't build walls in some cities in Civ4

[Slorrin]

Why is it that in some cities i don't have "walls" as an option. It says i can build a castle, but i require walls, so its grayed out. But i can't build walls. Is there some tech or age or enemy trick that can make it so you can't build walls?

lemme know if this is a bug or part of the game

 

[Clam Spammer]

Walls can no longer be built once you have Rifling, and castles once you have Economics. Kinda strange when you get that window where you can build castles but not walls, but that's just how it is.