Simply the castle building RTS
I know a lot of people if not all play RTS games for the challenge and with that they like to compete online. I however play RTS campaign's and care for story and emersion, challenge comes second. Stronghold delivers great story for a RTS and it is challenging at the right times, although castle assaults can be very hard.
After the campaign is over you still have a map builder and skirmish games that kept even me addicted. At the time this game was released it ran so well that it could handle a ridiculous amount of troops on the map, which made for epic battles. The map editor is time consuming, but with a healthy amount of time you can almost make anything. I don't even want to count the hours I spent recreating battles I read from books like Lord of the Rings.
Stronghold is a deep and immersive game, but it is as detailed as it is time consuming. It can take full hour or more before you got your castle fully running and producing troops. At the time that wasn't a problem however, for I had al the time in the world.