St. Mary's College was a women's student-teacher college which merged with Bangor University in 1977. The main building was built in 1906, with student accommodation added at a later date, It closed in 2008 and has been largely abandoned since, with a large fence added around the parameter recently.
The security on this site seems to be quite good at sealing up the access routes, a couple of days after my explore, there was a new 12ft fence up behind am existing 12ft fence, and steel boarding on the windows.
Also made a video of the explore, which can be found here;
http://www.youtube.com/watch?v=14x66oG6bso