A fine balance of resource management and combat - what other game do you have to make sure that archers on the wall protect the bakers running to get flour to make bread to feed your archers?
The AI of this game, though not exceptional, still does the job very well - and each "personality" has enough quirks where you'll start to look forward to going head to head with them.
This game is worthy to stand alongside the Age of Empires series, and I'd have to say Stronghold has that little bit more of a personality for me to choose it over that series when I want some old fashioned RTS goodness!