Currently building powerful Wall Towers is possible without building Walls. The AI enemy is often using this to create a "forest" of wall towers built instead of regular towers, which looks strange. But why not use the regular, and weaker towers for this purpose, and allow building the Wall Towers only within a wall? In my opinion wall towers are overpowered as the are implemented right now, because it is difficult to destroy them with siege engines, but they can easily be built. This can be deferred by only allowing to build Wall Towers within walls.