A lot has been said about this game in the other reviews, but it really is a great game. A basic description would be castle building, defense, and attack. The main campaign is a series of missions to consolodate control of the country by besting four other lords. You start out with small defensive oriented missions and grow to full-out castle assaults. Not only must you actively defend your castle, but you must build up your economy and keep you people happy.
The very first thing you notice is the attention to detail. Every building has a nifty little animation that goes along with it. The game is 2d, but that allows for a TON of fancy little animations. The trees all sway in the breezes, the rivers flow peacefully by and dairy cows amble about. More importantly, the archers make great little pincushions out of their targets and pit traps are suitably messy. Stronghold provides one of gaming's greatest thrills when a properly placed pitch trap is ignited by fire-arrow just as enemy troops arrive. FWOOSHH!!
Once you are done with the main campaign, there is a secondary economic campaign. This one is primarily to test your management skills, but does offer a challenge. In addition there is a wonderful editor for creating your own levels. Back in the day there was a thriving community making new castles to assault and defend, most based on real world fortresses. You can probably still find these if you know where to look.
I loved this game and heartily recommend it to anyone interested in castles and life in the middle ages.