The engi is the backbone of the team. He can heal, teleport and defend crucial areas. But he is low on health, and is easy to kill for a good player. Spies also tend to target this poor chap so it is very important to know how to defend yourself. When you are building things, it is alwas a good idea to get it up as qucikly as possible. When you are putting up a teleport, quickly move to the place where you want to place the exit. Also, it's not a good idea to place the dispenser and the sentry next to eachother. Not only does it make them both an ideal target for any nearby enemy demomen, soldiers or spies, but your team must go further back to heal and resupply. Always try to put your dispenser near the front and the sentry near the point/cart/hill. That way it is the last defence for your team if they can't stop the enemy. Also, if the enemy pop an uber it will most likely have worn off when they get to the point/cart/hill and your sentry will blow em sky high. Teleporters should be placed not to close to the front, but close enough so that your team doesn't have to run their asses off. Also, try to upgrade it as soon as possible. A fully upgraded sentry is good, but a medic/demo team is better. So keep your team close to the front with a dispenser next to them.