Great thing about this game is that no one can be attacked while being offline, and that that our cities and army (or what's left of them heh) are there when we log in again.
About replay value, well everything gets repetitive after some time, but
we have here few castle designs for one race, different regions will have differently looking castles so attack/defense strategy will be a bit different too. Also they say there will be really a lot of quests etc. so no worries it will be surely interesting.