buddy, for castles check out http://www.celticcastles.com/
for a list of castles you can stay in ( scroll down to the bottom right for the hostels: Carbisdale is 13.50 pounds a night, so they seem pretty reasonable)
Belford hostel (in Edinburgh) seems to be the only church hostel i can find.
there are quite a few websites dedicated to ppl who want to stay in lighthouses, but most of them are quite expensive (and alot in the USA/ canada)
In scotland check out http://www.scotland-info.co.uk/rr-house.htm
for 9 pounds 50 for a 4 bed drom in a lighthouse
you can also try a search for convents and monestries (there seems to be a fair few in Italy)
Hope that helps