[Kromm]

In my ongoing campaign, Architecture is the skill of knowing how buildings are put together so that you can blow them up (IQ-based roll), find secret rooms (Per-based roll), or navigate them even if you haven't been there (Per- or IQ-based roll, depending on whether you're looking or guessing). It's essentially part of the "extended suite of Survival skills" for built-up areas, where Urban Survival is the streets/exterior part and Architecture is the rooms/interior part. A Per-based roll against it can spot – or an IQ-based roll can deduce – where to find X, whether X is an elevator, an exit, a staircase, or a supporting member.

Strangely, I've never had designing buildings come up in any campaign ever, in any edition – not since 1986. Even back in my earliest dungeon crawls with GURPS First Edition, it was basically the "find hidden rooms" skill. That might have more to do with my crowd than with the game, though; I've never gamed with anyone who was into long stretches of downtime used to raise armies, buildings, and stock prices, so I've always strived to give the most sedate and academic of skills exciting adventure applications.