Well, there are multiple cannons and 2 fireplaces, the doors are the same as Hyrule Castle's except for the snowflake crest. There are multiple flags hanging on the walls with the Hyrule crest. with all the holes in the cieling, walls and floor i'd bet the reason it was abandoned was because they were under attack from someone. Another thing though is those giant cages which seem pretty mysterious. Also it looks like the room where you get the Big Key is a church or gathering room of somekind, though since they live in the mountains there couldn't have been enough people to fil the whole room.

EDIT: The cages are in the mini boss room and the other 2 are up the spiral walkway that have the 2 Ice Breathers in them. Also it makes me wonder, the doorsa to the mini boss room are noticably bigger then all the others, so I wonder if maybe a large beast was once kept in there.