Can criminals really take sanctuary in a church? Could they ever?
No and no.
The right of sanctuary was originally established in ancient Israel. There are passages at the beginning of the I Kings that describe taking sanctuary by clasping the horns of the altar (upright projections at each corner of the sacrificial altar). (It was tried twice, but only worked once.) There were also “cities of sanctuary” where a person might flee the wrath of the king or flee to self-imposed exile from a crime calling for ritual punishment.
The notion of sanctuary made it into the church. It had quite a few limitations on it: I believe that it could be used only for political crimes, not run-of-the-mill theft or murder; it had limits on how long the sanctuary could be invoked; there were religious penalties (such as excommunication) for its abuse.
A very brief treatment is given here:
http://www.newadvent.org/cathen/13430a.htm
Tom~