Hey, welcome to DOF.
First of all, town sieges are
in as of a couple of weeks ago. You don't need to worry about being offline; you won't be attacked, ever, unless you're logged in.
Building your city takes time, especcially since town sieges have been added. It dosen't take too long to get your town up and running with all the basic buildings you need; but to have a "finished" city, with multiple layers of walls and a dozen or so towers, it takes a while
And yes, it's just one really big server.
If you ask me, I'd say either get the game (because it's awesome) or wait a while for more content to be added and more bugs to be fixed. Just depends on how bored you are, I guess.
Anyway, hope to see you in the game.