Blueprints, which can be traded, are a way to speed up the building process -- or rebuilding, if enemies have destroyed your structure in RvR. Players with the appropriate skills can create blueprints of an already finished structure or through the architect's interface as a plan for a future structure. Through RvR, players can claim buildings built by others, either by deconstructing the entire thing or just capturing it; those structures can then be repaired or rebuilt by the new owners.
Want even more information on building a fortress in Camelot Unchained? Then check out Massively's interview with Mark Jacobs.