Placing high external walls around a town?

I have a question, I want to put high externals around a town to keep players out.. The issue obviously it will not let me "even being an admin" to place the high externals or anything else around it. Is there a way to override this so I can place anywhere? Thanks.

 

Usually I will build towns in the water using the Build plugin. This helps control this issue, however you can also use this plugin to forcably plant high external walls too. It may be tricky though, as it won't use the snapping function.