If I wanted my map to have a medieval look, I could make fence pieces that have battlements.

There are a few ways I'm thinking this could be done:

• once you design a piece, you could have a button for placing as many of those pieces as you want, but you would need to maintain that factory so there is a space-cost associated with it. (more casual)

• each piece you place needs to be actually fabricated. (less casual, more of a late game reward)

• user generated content library. Players could upload and share their designs to an online database and then you could pick whichever design you want to build. There could be a currency involved? Probably a little too convoluted.