The campaign is based on a home-brew system, partially concocted from a full blown RPG developed by someone at RPI with a very elegant combat and magic system. It was also incredibly complex and impossible to run smoothly without hundreds of hours of experience, so I simplified the combat mechanics, replaced the magic and clerical system with my own, and added percentile based skills. There were no arbitrary character classes; instead, you traded experience points for fighting levels or for specific magic spells, and casting the spell successfully depended on your intelligence. Those who concentrated on fighting (were exclusively fighters) had added attacks at higher levels. You could also become a cleric at any point in the game, where again you had to split experience points and were restricted to the spells for that religion (or you could split experience three ways, if you wanted spells, clerical levels, and fighting levels). Clerics also participated in rituals at regular intervals (every three months, with one major ritual a year) where various things would happen and you would usually walk away with some clerical magic items. The system worked, but sometimes not very well.
The main reasons for using a home-brew instead of a standard system was, in order of importance:
1) I like writing games, even not very good ones
2) I remember things I create better than something I've read
3) Some vagueness in the rules resulted in the players concentrating much more on the story line rather than optimizing everything (a real problem with your typical board gamer, and a dismally large problem with Caltech board gamers)
4) The players all had significantly more time to devote to the game than I did, and I needed something to keep them all a little off balance to keep the game from becoming an exercise in rules sleazing, which is really a subset of reason #3