On any map, the enemy Engineer is your first prime target. You must kill his teleport to cut off the constant stream of enemies and kill his sentry in order to proceed into other areas of the map. Killing a teleport means the enemy must walk to the frontline, wasting time while your team captures an objective.
On cp_Egypt, 2nd round, you can have an Engineer sentry jump on the arch and build a mini base on it. Correctly placing a sentry there makes it very hard to capture the 2nd point as most ubers cannot reach it. However, you must have another Engineer building on 1st point to stall enough time for the other Engineer to build up.
<-- This guy is TOTALLY not a spy.