Placeable buildings which a colonist can use to shoot from
Add windows.....
Yeah, adding gun loop segments to the wall would be nice so that you can shoot out.
We could use a wide variety of walls. Ye basic walls, Fortified walls, flimsy indoor walls (that look nice in a number of different cosmetic patterns), Gun loops, High tech force field walls, and so on.
I'm going to steal the unnoffical job of no one in particular by saying that you should of used the search bar as this has been discussed several times with a lot more detail than what is seen here.