Walkable but unbuildable terrain?

I'm working on a farming system that requires workers to be on fields in order to produce food. How can I make these fields (which are structures) block building pathing, but still allow units to be in that area?

Use a tileset that doesn't allow buildings to be built there?

If the fields are structures, use the circle of power pathing.

Worked like a charm. Thanks, Eriond.