I think an interesting upgrade would be a moat around the keep/tower and a draw bridge of sorts to get across.
As a 2nd upgrade you can put various fish/crocks in the moat that agro on enemy only.
When the keep/tower gets capped the moat disapears and it turns back to ground again till its upgraded.
www.infowars.com and also lookup Agenda 21
www.graystatemovie.com its coming if you like it or not.