This is a new drawing, re-creating my 1980 original from page 14 of the Advanced Dungeons & Dragons adventure Q1: Queen of the Demonweb Pits (TSR Hobbies, 1980). Check out my latest Kickstarter project to re-create my old TSR art: [link]
This is the castle of the Caer Sidhi, if I'm remembering the name correctly. This is one of those understated pictures that still manage to inspire a lot of ideas. The look is strange and otherworldly, very alien and unsettling. It perfectly suits the sinister nature of the inhabitants of the alternate plane where it's located.