Thirlwall was built in the 12th century, with the usual frequent additions.
The history of the place is sketchy. Evidently a John Thirlwall was the local baron. He went to fight in the First Crusade and did rather well for himself, returning with various treasures including a reputed table of solid gold. When he went off to fight in the less successful Second Crusade* the castle was left lightly defended. Raiders from north of the Wall came down and sacked the place. One family retainer was a dwarf. Determined that the plunderers would not get the gold table he chucked it down a well. Then realizing that he would surely be tortured until he 'fessed up he tossed himself in for good measure.
The dwarf, evidently now invested with some magical powers, is said to still be guarding the table deep underground.
I actually had to make a second trip to Thirlwall castle the next morning. The young son of my hosts had lost a toy-a small stuffed turtle-earlier in the day, perhaps at the castle. I looked high and low for it and found no trace. I seriously considered the notion that the magic dwarf had nicked it, but I understand that the turtle was found a few days later behind dad's computer, having fallen off a shelf.
To visit Thirlwall. Well, if you are walking Hadrian's wall path this is a very nice stretch and the path takes you right past it. If you are in Greenhead you need to walk along a rather dreary path north from town between the railroad tracks and the river Tipalt, which is running in a boring straight ditch. It's about a mile from town.
*Yes, I am aware that there was a 50 year gap between the start of these two Crusades, but "I tell the tale as I heard told" by a local. Besides, if you have a problem with that then the whole Magic Dwarf thing isn't going to work for you either!