Just thought I should add some of the other classes. Im not an expert but i know my way around a wrench fee free to add and disagree that how we learn. Also feel free to ask stupid questions. Also this is a work in progress.

engineering 101

So you got your shinny yellow helmet and a brand new wrench unfortunately all those books don't prepare you to the hell that is out there.

Engineering is hard constant work.

The Golden Rule

The most important thing in my opinion is the following fact. After you set down the blueprints if you hit it whatever it is will build faster in fact the more people hit the object the faster. nothing funnier than 3 engies building a dispenser.
This might seem trivial but remember this is precious time to let you dispenser store metal or getting your gun to a higher level that will give time for people to regroup or hold the enemy back or getting more people to the where they need to be. All rules have exceptions, there are certain times where it is better to just let the buildings let them do what they do. For example when building in the courtyard in 2fort you have a resupplier right there. So build a sentry gun (SG) resupply build a dispenser resupply and start upgrading and run back as needed.

An engineer is more than his SG

Its easy to hide behind you gun and pray to sponge bob. Thats the difference between an ok engineer and a slightly better one. Shotguns can take out spies in a few shots before a spy can get to you or finish a scout or anyone really. Its also a great way to get peoples attention and lure them to your handy SG.
Ive found the pistol is possibly more handy than you might think. The pistol is surprisingly accurate and annoying. apart from extra ammo to finish someone off instead of reloading your shotgun it is better than your shotgun to shoot someone who is far off. So anyone out of the range of the SG or if you just want to annoy someone in the distance such as a sniper. Also don't be afraid to walk away from your sg to dish out some damage.

Two schools of thought

Ok here is where things get messy. In my opinion there are two kinds of engies, defenders and attackers. The line gets blurred at times but i think this example will help. If you build your fort (fort = Sg and dispenser near by so you can bunker down) in the intel room in 2 fort or in the courtyard you are a defending engi if you are building in their base your attacking. For the most part in maps like 2fort dustbowl (red team) gravelpit (red again). For Campture Point (CP) maps like grannery or well an attacking engi is far more useful. Heres why: building a whole CP behind is just giving your enemy ground. This leads to quick defeat or at least a very long game which ends in sudden death or just lots of grumpy players. Attacking is frustrating on a good day and soul crushing on a bad one. When attacking the rules change alot. For the most part your job is time management, these maps tend to be large and much walking for heavies leads to them having shattered knees and thats not good. So teleporters are a must its hard to know where to place them and often change on the situations. For the most part build it a room or a good 30 meters behind the front lines. The important thing is to stick your exit somewhere reasonably hidden or at least out of the way to avoid spies or anything with a gun. While on the subject of teleporters as your team moves forward so do spawn points and your team so welcome to the 5 key on your menu. This will destroy whatever you have built.....sad i know but winning is better. same thing goes for your dispensers and SG. When attacking things might not always go as planned so the important thing to remember is that not all SG have to get to level 3. Crazy? i know but a 1sg can stop a scout rush and 2sg can stop most everyone. Why not build a 3sg well sometimes you don't have the time or metal, but a 1sg or a 2 sg can buy enough time for people to get back. Another useful tip no good engi is above scrounging for metal. Dead enemies or allies will drop their guns......pick them up each weapon is 100 metal thats a dispenser.
Its tricky my suggestion think about your enviroment (map, situation, team, number of other engis, etc.) and think about what kind of engi you should be. Get familiar to where metal supplies are on the map. For the most part I think attacking engis do a lot more for the team.

Spies

Your worst enemy and biggest source of headaches and frustrations...seriously i wake up in a cold sweat at night with images of sappers. Here are some tips as to dealing with them. first of all and perhaps most valuable and obvious keep your eyes open! Not all spies are 007 (despite other posts on this forum) at some point they will have to either cloak and/or uncloak you'll be amazed how often you might catch one out. For the most part take the sapper off the dispenser 1st. A 3sg will have enough hit points to take some damage your dispenser doesn't. Unless you have plenty of metal nearby ie a resupply room or a big metal box somewhere your dispenser is more important. Im one of those who think that dealing with a spy who is constantly sapping you is not your problem some one in your deal has to kill him. Otherwise at some point you will lose your objects or get killed. For the most part this is a situation you want to avoid so some more tips for dealing with spies. Paranoia is not only fun but you never know you might catch one out. Try to keep a 10 second memory to the traffic if you just saw Xhello kittyX run off into the madness and you see him return chances are its probably not him/her. A mediocre spy wont change their costume too often so for a while you will see the same person over and over making them easy to kill. Spies have ability to respect the laws of physics and are unable to pass through you or your gun.....how can you use to this your advantage? Well anyone jumping over your objects are probably a spy. Want to one up them? (this is a WolfDevil exclusive tip honestly havent seen anyone do it)If you put a sg on the door way near the stairs closer to the supply room in the courtyard in 2fort (if anyone can give me an image might help) and if you stand on top of it (jump and duck) there is no way for them to get behind you so just shoot them in their bald spot. Want the absolute best tip? befriend a pyro or a demo or a heavy. Also keep your ears open a good team will constantly give out info about who's a spy


Tips for other Classes

This is not called Team Fortress for nothing. Engis need help at times. Here are some tips that will help your engi, after all you will at some point or another run to a dispenser for ammo or when you are engulfed in flames. Or may be you are just lazy and don't feel like walking. How to help? Spies who are sapping objects are your responsibility to kill your engi are far too busy getting rid of the sappers. Although heavies have the top priority on teleporters sometimes engis need to get to their objects in order to repair them let them. Engis need metal to build the goodies so try not to take the metal if you can that also includes guns that have been dropped by your recent victims. Keep them alive and they will return the favor.

Summing up

Its not easy to be a good engi but its not easy to be any one. Now you have some more info that will help.
Hit your objects they will build faster
Dont be afraid to take your guns out and teach them a lesson
Not all SG have to be level 3
Spies are evil but if you keep your wits about you you should be able to handle them
Spies cant pass through people or things
Familiarize yourself with metal sources
There are many exceptions to the rules and nothing beats being able to think on your feet. Don't be afraid to experiment with crazy ideas you might get a good one. Despite all of this experience is the only way you will get better.

More to come but its late and I'm all ranted out again feel free to teach me a lesson or two and if anyone has images or maps or willing to take pics for better illustration will really help. More to come including how to repair and where to build ne who thanks for your time.

couple of things to add wolfie;

Engineers can ROTATE things! it's brilliant. If you hold down the right button then your object will slowly rotate 90 degrees. If you click it, it will immediatly rotate 90 degrees. You can keep doing this round and round. Now this is very important for 2 devices:
A> the sentry. A sentry covers 360 degrees BUT it turns slowly. A scout (and a pyro as well) can completely take out a lvl 3!! sentry if he/she can sneak up behind it and start running round it at close range. Therefore it is most essential to actually have it pointing in the right direction (i.e. expected entrances). In addition to that it is also a good idea to have the sentry backed up on one side to a wall so that they get blocked when attempting to run round it. This also lets you put sentries facing the correct way into tight corners by right clicking twice making it face you (examples of useful places for this are when defending in dustbowl and you want to put the sentry in the corners of the stairs).

B> the teleport exit. This is soooo damn important. Think to how many times you have gone through a teleport and come out the side facing a blank wall thinking "where the hell am i?" you then spend valuble seconds trying to orientate yourself. the reason this happens is because the engineer has just placed the exit near the nearest corner and left it at that. When you come out of the teleport you come out facing the same way the engineer was facing when he dropped it there. A considerate engineer will rotate the teleport exit by 180 degrees so your facing away from said wall. If you're in tunnels or something please make sure you're facing as many recognizable land marks as possible so people know WHICH tunnel they are in (especially important for instance if you're in the sewers at 2fort or something to know if you're in the enemy sewers or your own, etc).


also as wolfdevil said, but would like to emphasize it. The pistol is amazing. especially for spy killing. Admitedly you need an almost full clip to kill a spy but i find it easier to target people compared with the shotgun (as if you miss one shot with the shotty, you've lost a big chunk of damage). But yeah it is down to personal preference. Also something DoityWottenBastid pointed out was that if you've got a fort, you should (if you feel safe) greet people with a wrench. Going a bit away from your fort and wrenching people head on gives you a definit spy detection because he will bump you. And as you're greeting him head on he wonte be able to backstab you (well i say that but everyone's encountered the dreaded Frontstab).

And one more chunk of text, especially important for the starting engineer. The placement of sentries:

You need to make sure your sentry is not able to be shot at from range. Snipers and soldier should not be allowed to see it, otherwise it will just go down in an instant. A very important tip is that if you increase the model detail of the game to high you will actually see a sphere/dome representing the sentry range before you place it [See Fig 1]. And now for covering corners. I believe The biggest non ubered threat to your sentry is a demoman, much more lethal than spies, because they dont have to come up close. Now out in the open there isn't much you can do (shouldn't have it out in the open really anyway) but when covering corners or corridors there is something you can do to make it more difficult for them. Admittedly this is open to debate but if you'll look at Fig 2, you'll see a wonderful paint drawing i made of a corner. With a blue and red sentry. I believe having the sentry at blue position makes it harder for a demoman to take the sentry out than at red. The demoman relies on quick poping out and laying a mine in order to take your sentry out. By putting it on the blue position he has less safe space. On the other hand sticking it in red means once you heard the sentry you're already fully exposed and thus ...DEAD. Also it means you can go pop a cap in his ass while he's setting up his web of mines.

So two different approaches and really need more input from other people as to ideal positioning. Guess it's down to personal preference. I'm slowly being convinced of putting it at red. I play a lot of demoman and to be honest a determined demo, who knows about the sentry in advance will take out you're sentry guaranteed. only thing you can do is kill him or if you have a heavy around (or lots of ammo) is shoot the mines away. At the very least, if you've seen three or more mines laid at your feet, run and save you're life to quickly build up another fort.


Oh and there are some interesting videos on youtube on getting to strange places using teleporters and dispensers:
Getting on the roof of B on gravel pit:
http://www.youtube.com/watch?v=vXDcQpVct8U

Getting to the top of Cp1 on well:
http://www.youtube.com/watch?v=VkzbpI7VKAM

Oh and also, it is important that the first thing you lay outside of spawn is a teleport entrance. You will eventually need to build one and it saves having to go back an do it later on in the game.

Oh and also, if you have a number of engineers on the team try not to cover the same spot that is already being covered. (very few instances where this should happen such as on dustbowl) and repair each others stuff!! WORK together. thats one of the best things about engineers.