If you think the old Armory at 14th and Mission streets — that giant brick fortress built in the 1910s to look like a re-created Moorish castle (why not?) — is imposing from the outside, check out its insides. Once a month, Kink.com, the alternative porn studio that bought the enormous, spooky building in December 2007, opens its doors for free public tours. You'll get to explore the Armory's authentically dank interiors, along with its fabricated but equally dark film sets, including fake interrogation rooms with BDSM restraint devices. Sorry, you don't get to watch any sex scenes in action as part of the tour. You'll have to visit one of Kink's 17 fetish Web sites to see those.