Can't build ancient walls in town; why not?

[MIS]

It's not giving me an option to build them, and I have masonry. I can build them in my capital (in my only city). A fort town says it gives extra healing/strength to them. So I'm not sure what I'm missing.

I went back to a previous save, before my neighbor declared war, and I could build them then, so I'm now I'm really confused. I looked on "View Hidden" and it didn't even come up. And I think I had enough money.

 

[Leyrann]

You can only build walls on the center tile of a town.

Also, you cannot purchase walls and a unit in the same settlement in the same turn. If you buy one, the other disappears from the purchase list.

 

[CliffCo]

Is your town under siege, as that will prevent you from building any.

 

[Yzman]

Is your town under siege, as that will prevent you from building any.

I think if an enemy is right next to your town in general it won't let it build. I know I can build them when an archer is attacking, but not a melee.

 

[Taefin]

Same with not being able to build galleys when an enemy boat is adjacent (I think).

 

[AriochIV]

I get the feeling that the town also has to be a minimum size, but I'm not certain.