Next random question, if I may.
I don't really "see" PD?  I tend to buy 20.  Are they there immediately, but I can't see them unless attacked, and then only in battle?  Do they slowly "develop" --- occasionally I've seen a few odd units apear that I don't recall building, I think, but never like 20.  If I take those out, am I taking the PD I bought?  Does it regrow?  That kind of thing... ? 
