You know about houses, roads, and other built environments. The topics you may know about include aqueducts, bridges, buildings, worked caverns, fortifications, roads.
Synergy: 5 or more ranks in Knowledge (Architecture & Engineering) gives you a +2 bonus to Search checks involving secret doors and built environments. Also, 5 or more ranks in a Knowledge skill related to the topic being investigated gives you a +2 bonus to Research checks. These bonuses increase by +1 for every additional 10 ranks (that is, +3 at 15 ranks, +4 at 25 ranks, and so on).