It is important, as an Engineer, especially on defense, to make sure you have a teleporter exit and entrance active before a round begins. You can get all four of your buildings online before the round starts and should strive to do so before getting your sentry or other buildings to level 3.
Teleporters greatly reduce the distance your team needs to travel to get to the front lines, and you rely on them keeping the enemy team off of you and your buildings. Provide for your team, and they'll provide for you.
Featuring Uncle Dane as Col. Buddy McFartface