Stronghold is a mixture of the simulation and tower defense genres. This actually really helps both, as simulations don't tend to have a very strong demand for immediate response, making many of them a bit slow paced. Tower defense games are always a bit repetitive and tend to boil down to only a couple core strategies. Stronghold combines the concepts into one game where each covers the other's weaknesses quite well.
Just when you feel the monotony of the sim style castle building set in, here comes an approaching army to keep you on your toes. And while the sieges would get tiresome and annoying as the best responses are usually pretty obvious, the way the simulation part restricts your resources creates an interesting challenge for supporting your plan forcing you to make compromises.
It's not the best game on this site, but if you like building clever defenses, and enjoy interesting challenges, Sronghold is for you.