So I was bundling around exploring in a new world, and I came across an ordinary looking fort. No doors or anything, so when I broke through I was expecting it to be an empty shell.
Stairs led to the top, and at the top was a chest with some good ores, but that was nothing compared to following the stairs into subterranean level. There are acres and acres of rooms, traps, jail cells, and lots of chests with mithril, gold and silver ores. I only took a few screenshots but it was an exiting find for me 
