[ASK] What is the lowest level TH where you can have a "obstacles only spawn on border" base?

At what TH can you, using all your walls and buildings, make a base without any 2x2 empty spaces? At what TH can you make one counting decorations, and if it's any different what TH without decorations? So obstacles can only spawn on the 3 tile wide ring around the base.Please don't count gravestones, as you can't really control where they go.Thanks!