Torwood Castle is an interesting 16th century ruin, which now stands neglected in an overgrown field near the village of Torwood. Some rusty scaffolding showed that restoration work had been started at some stage, but it looked like nothing had been done for a long time.
When I visited in September 2012, the site was unsecured, and nobody was around, so I had complete freedom to explore the outside of the castle. Unfortunately all the entrances to the castle had been securely closed off, so there was no way to get inside.
You can learn more about Torwood Castle at the Undiscovered Scotland website.