Mount Fortress, or Fortaleza do Monte, is overlooking the ruins of St. Paul's and is the home of the Macau Museum. There is a park on top, with great views of Macau.
It was built in the early 1600's and played a big part in holding off a Dutch invasion in 1622. It stopped being a military base in 1965.
Unfortunately the conditions weren't very good for taking pictures last time I was there with a camera. So I only have a few, and they are mostly overcast. I'll try to remember to take some better pictures on a future trip. But for now, here are a few of the better ones.