Your buildings will help assist you and your team on winning the game but you need to know your buildings and what they do as an engi.

The Sentry: The Sentry gun is an area denial tool to not let enemy players in a certain area. (note that spies are not detected when cloaked and/or invisible) the sentry is target #1 for enemy demomen, soldiers, and spies so make sure you protect your sentry with your life so it can protect your team. Sentry costs 130 Metal

The Dispenser: The Dispenser will provide health, ammo, and scrap to friendly players, it's job is to provide a place to retreat and heal from the frontlines then get back in fast. the Dispenser cannot defend itself like the sentry so it's even more vulnerable to enemies then the sentry. Dispenser costs 100 Metal.

The Teleporters: The Teleporters will provide a quick way to get closer to the frontline they are one way though and are in specific order (entrance and exit) (note that enemy spies can use teleporters for their advantage) Teleporters should be placed near key areas to help your team such as the frontlnes and the spawn.

I recommend you aways build dispenser first if your nest is far away from the frontlines but if it's closer build a asentry first. teleporters should be last priority.

Remember that Ammo boxes not just give you ammo but also scrap to use for your buildings you have a max of 200 scrap to use for buildings

While you are not usually a frontline class you still need to know how to defend your self in case of a spy coming to destroy your nest, you have tools to defend yourself

1: The shotgun, this is your go to weapon and your primary for dealing damage to enemy players, you need to be close to do good damage

2: The Pistol: The pistol is good for medium range engagements or to weaken enemies before finishing them off with the shotgun

3: Wrench: if the enemy is right in your face or if you are out of ammo, it does a good amount of damage to enemy players but it is the most risky to use