Although not very realistic, I thorougly enjoyed the medieval castle building sim called Stronghold that came out a couple of years ago. Who could not like a medieval game that lets you catapult dead cows into a castle.
Although I played through the campaign a couple of times, it has been long ago enough that I may drag it out and play through again.
Anyhow, if anyone is interested there is a Stronghold 2 in the works and this time they are taking the game full 3D.
Here's a link to a preview:
Stronghold 2
PS. You can pick up the original game in a budget re-issue for $10 or less, and I have seen it packaged with Stronghold Crusader which was a stand alone expansion for less than $20
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