I'm quite lucky to have several castles, and a load of old churches and manor houses, all within 10 miles of my home in Leicestershire. One of my favourites is Ashby Castle. Until the 15th Century, it was little more than a fortified manor house. Then King Edward IV gifted it to Lord Hastings, who set about giving the I'm 9th Level build so I'll build a stronghold treatment.
Although the castle is now just a ruin, visitors can still climb 78 feet to the top of Hastings Tower. A later addition to the castle also survives in the form of an underground passageway, linking the kitchen and the tower. This dungeon isn't always open to the public, as it can get a little flooded, but luckily it was last time I was there. Only minimal artificial light has been installed for safety in the entrance chamber, and spending just a few minutes down there makes it easy to imagine you've just wandered into Stonehell. Another thought that struck me was how narrow the corridor was. It makes me wonder if dungeons with five foot wide corridors would be likely. Also, underground chambers would be fairly cramped places, with lots of irregular architectural features for nasty things to hide behind or in.
Here's my pictures of the castle. The ones in the dungeon were taken with flash, but they still give a sense of how dark it was down there (my wife and daughter are standing about 25 feet down the passageway in one of them, and are barely visible). I'll snap some pictures of some other local buildings as I revisit them throughout the year.