Engineer

Engineers define the speed of pub play. A well placed sentry can halt an enemy attack, until a Demoman or a Spy comes along and takes you down. You also funnel people up to the front line with teleporters, and keep your team equipped with dispensers. A few good Engineers can defend a capture point by themselves, and can stop any chance of enemy victory in other styles of play.

GENERAL STRATEGY

Good sentry placement is how you stay alive. Give yourself a few vantage points on places enemies may cross, but keep it out of range of grenades launched by a demoman. For all intents and purposes of most games, your sentry is your weapon. Even if you have to go out, lure enemies to your sentry. There are a few reasons why this is a better strategy than using your shotgun or pistol:

1. Sentries never miss. We all are human. We all miss often, especially those of us (myself included) who have subpar PCs which lag a tad during combat. I trust a sentry's aim over mine, merely because it locks onto enemies.

2. Sentries, at level three, are rated to put out 8 bullets per second, along with rockets. Along with the knockback, this is enough to turn any enemy into a leaky piece of meat very quickly. The firepower of a level three sentry is unparalled in TF2.

3. It is difficult to kill a sentry, unless you are a demoman, if being shot by it. Beyond the psychology of the beep followed by the knockback, your aim and a short time to live make it difficult to kill a sentry. Even if they come close, a couple of knocks of the wrench should put it back up to speed.

A couple of concerns with sentries:

1. Don't place them at the end of long spaces. They do have a range, and a Scout could just whip out a pistol and take it out bit by bit if they get a shot at it.

2. Don't place them immediately around corners. Pyros can caddy-corner their flamethrower and burn it to the ground.

3. Don't stand in front of it. Your sentry can hurt you if it shoots at you. This is not true of your team.

4. Don't block anything important by it. You can't walk through your sentry, so don't use it to block off anything important to you. The enemy can't walk through it either, which can be used to your advantage as discussed later.

Dispensers will often make or break Arena rounds, and help greatly to yourself and others in rounds on other styles of play. On Arena, they give health, which a lack of healthpacks and difficulty in getting Medics to play make important. Elsewhere, they give ammo, which is needed by yourself for upgrading/repair, Soldiers and Demomen becuase they run out of ammo quickly, and other classes to bolster their ammo count.

Teleporters will often act as the deciding factor in CP push maps. The ability to get your team immediately to the front line gives you a distinct advantage in defense and offense. Put it somewhere useful, but not out in the open, as they can get shot and killed easily. If one teleporter in the pair gets destroyed, the other reverts to level one, so be careful.

As I alluded to before, buildings can block off entryways. This can be used to your advantage. Place a dispenser at the beginning of a hallway, and a sentry covering it. Even spies cannot get past it, which slows your opponents down dramatically. This is a good strategy if other Engineers have pragmatic dispensers already covered, giving ammo and health when needed.