Stronghold

The Good
This game played wonderful on my P75 five years ago and plays just as well on my 500mhz machine now. This can be a simple game to start and slowly get more challenging. Does not get too frustrating like some of the more complicated strategy games.

The Bad
Sometimes I get really engrossed in the graphics and forget I have a whole kingdom to worry about :)

The Bottom Line
If you were to mix up Populous, Power Monger, Civilization, and Sim City. Then throw in some classic SSI RPG elements. You would come up with Stronghold.