I’ve built externals like this before I can’t get it to work terrain is not the best but that hasn’t stopped me in the past so I don’t think it’s terrain. It keeps telling me I can’t stack building privileges or something. I don’t know if I’m building out far enough, I literally built out 10 and it didn’t work. It’s obviously got to be something I don’t know. Looking for a friend to help. I’ll even toss a small start if you’re willing to hop on and help me figure it out. 
#Need help with external TC’s (giving starts to who helps)
rain sandal
rain sandal
I build 2 triangles off my base then two squares I then place a 1x2 gate house and from there build 2 triangles and 2 squares off the gate house and then place two triangles for the tc along with half walls for stability and it is saying “can’t stack building privilege”