Building large stone wall problem

[bikersmoke]

Hey my first post is gonna be a dumb one, or at leat it feels that way. I may be reading the build tree wrong. I have a minor city with a stone wall, and want to build a large stone wall. The icon for large stone wall is beside minor city icon in the build tree so I thought with minor city could build large stone wall but says can not build yet. Is there something im missing that i need for the wall besides the city or am i reading the build tree wrong? I have played RTW all the way through and never had any problems with anything but just satrting MTW 2 and already cant read the tree right.

[Bob the Horsearcher]

You can build large stone walls in cities with a population of 12.000.

[bikersmoke]

TY, well im way off only have pop of 4100. I dont like the building tree at all seems like it has nothing I need to know.