I believe it doesn't matter if you buy it after the armor, but I'm not sure. If you already bought the eagle, and plan to re-buy him, don't get a moat anyway.
Yeha, ive started experimenting with moat though, normally i have 2 big armies and one is in my base usually, but with this i dont need as much of a big army in my base = more forward units etc. Only problem is i havent had anyone attack my fort yet while ive had moat.....so idk if it actually works ;P.