I imagine most confrontations taking place in a monastery would be very one sided, and very quick. Of course, that's true for the majority of 'battles' in the Dark Ages. For every Clontarf or Maldon or Brunanburh, there would be thousands of small raids on vulnerable targets, where the defender stood no chance at all.
But those battles are a bit boring to represent on the table-top, so we sensibly make the assumption that the defenders have had enough notice to gather their forces and move to a suitable location for battle. In which case a monastery makes a perfect place to defend!
I say go for it, not least because your terrain buildings are always of an exceptional quality and I want to see more!