For those who dont know who the engineer is?!

The engineer is the most played character in TF2 the engineer basicly build's sentry gun's for offense/killing (in more detail) - the sentry gun automatticly fire's bullet's at the enemy when there are in view, the dispenser is built for support - healing E.T.C + teleport enterance's & exit's are built for support getting you further on the battle-field, you need an entrance and an exit for it to be usable, all of these structure's can be upgraded with a swing of the wrench and some metal, a way of getting some metal is to either go back to base and take some from the medi cabinet or pick up any weapon's that have been droped when an enemy has died, the upgrade can go up to level 3. The reason for this character's being played so much is because you dont have to do hardly anything. all you need is to repair the structure's if it's a little damaged.

BTW: engineer's can only build each of these structure's once
(The video below is not all game-play footage)