A lot of good points here, but there is a lot more to the Castle Doctrine than someone breaking into your house in the middle of the night and points a gun at you. We should all know what it means in the state we live in. Factors such as time of day it happens can have something to do with it. Also, what if you invited the person into your home, or both people live there? I believe some states extends your "castle" to you vehicle. This was proposed in RI, but didn't pass. Your Castle may not necessarily just mean your home either.
Jim has provided and excellent write-up of the Castle Doctrine in RI and backs it up with case law. it's worth reading even if you are not from RI and it would be nice if we could get one for each state on the forum.