Knowledge (architecture and engineering) covers architecture (which race built a given structure, when it was built, etc.), and engineering (building siege engines, the best way to bring down a building, determining the soundness of a structure, etc.).
The PC can draw up plans for the building of a structure – anything from a simple hut to an entire city - and has a general idea of how they are built.
The DC for drawing plans varies, but a good rule of thumb is DC 10, +1 per 1,000 square feet of space the structure occupies. On a failed check, either the structure is too complicated for the PC to draw, or there is some flaw in the plans that likely won't become apparent until building is started – an incorrect measurement, a low estimate on building materials, etc. This generally has little effect beyond delaying building – a DC 15 check by an engineer can determine the flaw – but the DM can determine other effects as necessary.
On a DC 10 check, the PC can determine which race built a given structure, if the race is one common to the campaign world. If the race no longer exists, the DC can be increased to 15, 20, or more depending on how long ago they lived and how many examples of their craft remain, or if they were obscure (geographically limited in scope, or extraplanar in origin).
On a check of DC 10+, the PC can determine when a structure was built, based on its architecture. The modifier for this roll is +1 per 100 years of age, though 5 or more ranks in Knowledge (history) grants a +2 bonus to the check; if the PC has detailed knowledge of the race/culture that built the structure, he gains another +2. A successful check doesn't give an exact year, just a general idea – within 25 years either way.
On a DC 12 check, the PC can determine how sound a structure is – he can tell if a cave roof is about to collapse, for example, or whether or not the rope bridge will support the party's weight. This can be increased to 15, 20, or more if the structure in question has been specifically weakened without making it obvious – ropes partially cut through, stone weakened by magic, etc.
The PC can build (or help build) a siege engine. When construction first begins, he must make a DC 15 check; every 5 points over the DC grants a +1 bonus to his Craft check when building the device. If he is part of the crew of an existing siege engine (whether he helped build it or not), he can make a DC 15 Knowledge check to grant a +1 bonus to damage per 5 ranks over the DC.