Although different situations on different maps will have their own set of circumstances for either engineering role, the basic opening play should always apply. Leave your spawn by the most common route, build a teleporter entrance ASAP, return to a supply cabinet in your spawn to refill your metal, then head off to your position. You can set up your dispenser and sentry gun first if you want, but always build the teleporter exit somewhere out of sight as soon as you get the opportunity.
On that note, the fact of the matter is many engineers (perhaps half that I’ve seen) don’t even consider building teleporters at all until the game is all but over. I can’t stress this enough – build a teleporter entrance ASAP and return for metal! You lose nothing, and once you hit the front lines all you have to do is place the exit which only takes a couple of seconds. The teleporter is in many situations a more important building than the sentry gun, but the reasoning seems to be that sentry guns get more points, so they should take priority. DON’T let yourself think this way!
Engineers working together can be much more than twice as effective as a pair working apart. Whacking a building under construction with the wrench halves the build time, and having two engineers whack it cuts it down to a third. Taking advantage of this 2 engineers can build their buildings one at a time, in both cases working together to get stronger defences up quicker (quality over quantity). Any number of other benefits come from such teamwork, like erecting sentry guns in around 3 seconds, repairing and de-sapping friendly buildings to keep them active, and having one engineer repair while the other harasses attacking troops.
The engineer’s most obvious rival and counter is the spy. There are a range of counters the engineer has to the spy, but an engineer defending his gear should always follow a few simple steps. First, stay close to your buildings and be ready to knock away any sappers. 2 or 3 swings deals with them. Second, always be on the lookout for spies. Sometimes they can be easy to spot, sitting around watching you like a hawk or even assuming your name and running towards you. Be suspicious of everyone on your team (especially those with primary weapons), and give everyone who comes near you a solid whack with your wrench. Check out the Spy Detection guide and Engineer, Versus article for more on this subject.
A common tactic of good spies is to plant an electrosapper on your sentry gun, then quickly stab you in the back as you go to wrench it off, since your buildings are disabled while being sapped. Avoid this by making sure you never turn your back on a spy, circling around your building while de-sapping and laying into the spy with the wrench if the opportunity arises.
The buildings that engineers construct are impassable to themselves and their enemies. This means you can crouch jump up onto your buildings and use them as stepping stones to reach otherwise unreachable areas. One great example is getting on to the roof of control point B on Gravel Pit using a combination of dispensers as platforms and teleporters to get up higher than you can jump. Search for it on YouTube if you don’t know how.
Your buildings make excellent cover against as enemies as long as you have metal. Crouching and hiding behind them makes you difficult to hit, and any attempts to kill you will result mostly in damage to the building you’re hiding behind, which can be quickly repaired with a swing from your wrench. With a good sentry gun spot and a dispenser in range behind you, sentry guns can be ridiculously resilient.
Following that last point however, watch out for sticky bombs! 3 sticky bombs can level any engineer/building within a decent radius, so if you see them landing around you or your sentry gun’s feet, get out of there! If you want to save your buildings, the best course of action to take is to act quickly, rush forward at the demoman, and take him out with the shotgun before he can set up enough mines. He has pretty weak close range weapons, so this is a possibility if he is relatively undefended. Of course you could also try and shoot the sticky bombs away with the shotgun, though this is extremely difficult and unreliable.
If any of your buildings get just slightly damaged, it is usually a stray bullet from an attacker that hasn’t specifically targeted them yet. However it should be taken as an indicator that they are about to become targets once your enemy deals with whatever he was dealing with first. You should get over to defend them until the threat has passed.
