Basic Engineer strategy

The Engineer has the unique ability to construct Buildings. Although he possesses a weaker set of weapons, his Sentry Gun is more than enough to fend off multiple enemies. His Teleporter system sends teammates to the front lines, while his Dispenser heals and supplies ammunition. After constructing a building, he can pick up and haul it to a new location, allowing him to keep his base alongside an offensive push. The Engineer and his buildings will frequently be targeted by Demomen and Spies. A well-timed enemy ÜberCharge, in particular, can quickly ruin all of his hard work. When his buildings are destroyed, it is most important that the Engineer makes it out alive so that he can reconstruct his base. A good Engineer will place his buildings in strategic locations and not be afraid to get his hands dirty defending them.

Primary Weapons

Shotgun

The Shotgun is a deceptively powerful weapon at close to medium range. Use it to defend yourself when alone and scare off enemies that your Sentry Gun cannot target. Choose the Shotgun over the Frontier Justice if you need to deal consistent damage over time.

Frontier Justice

Barring magazine size, the Frontier Justice is statistically identical to the Shotgun. The small magazine size prevents you from going on a mini-rampage even if you have stored up a great number of Revenge crits. Thus, you'll get the most out of the Frontier Justice if you place your Sentry Gun in an offensive position, where it should be able to get a few kills or assists before being destroyed and granting your Revenge crits. Equip the Pistol alongside the Frontier Justice so that you can conserve your Revenge crits. Although you can destroy your own Sentry Gun and still receive Revenge crits, the firepower of your Sentry Gun is generally more valuable.

Secondary Weapons

Pistol / Lugermorph

You have a tremendous supply of Pistol bullets, so feel free to spam them at any enemies. Its effectiveness at medium range makes it combo well with Combat Mini-Sentry Guns.

Wrangler

Use the Wrangler to give your Sentry Gun extra staying power and allow it to attack enemies outside its normal range. Don't be afraid to allow your Sentry Gun to function normally; its AI is more aware and accurate than you are. By firing at yourself, you can Sentry jump to unusual locations.

Melee Weapons

Wrench / Golden Wrench / Saxxy

Hitting buildings with your Wrench will cause them to build twice as fast. When using the Wrench, the game prioritizes repairing buildings over melee combat. Thus, if you attack an enemy Spy next to a friendly structure, you will hit the building instead of him. Use your Shotgun, which deals greater damage, to dispatch nearby foes instead.

Gunslinger

The additional health the Gunslinger grants makes you a viable attacking force; use your Shotgun and Pistol to complement Mini-Sentry fire. The offensive playstyle complements the Revenge crits you can receive for your Frontier Justice.

Southern Hospitality

You're probably going to die if a Pyro corners you anyway, so the fire vulnerability is negligible. The bleed effect the Southern Hospitality afflicts is useful for tracking down Cloaked Spies.

Jag

The Jag will let you establish a new nest of buildings very quickly. Use it to quickly deploy or redeploy Sentry Guns at forward locations. It is a weak weapon, so use your Shotgun against close foes instead.

Buildings

Sentry Gun

Sentry Guns are best used for area denial, not killing. The difference between excellent and average Sentry Gun placement can be very small. When placing a Sentry Gun, note where it can shoot and where the enemy can fire from. Moving it even a few inches can keep it from being picked off by Soldiers or Snipers. Avoid "gimmicky" Sentry positions such as looking down holes in ceilings, in the middle of narrow hallways, or around sharp corners; a Sentry Gun is useless if it can't fire at anyone. However, placing Sentry Guns in well-frequented passageways, out of sight, can insure that the enemy team is incapacitated. Putting Sentry Guns in corners is recommended, as it allows your turret to focus on one 180 degree angle, instead of a 360 degree. Corners also help protect your turret from flanking, and allow a degree of safety from Sappers, though this does raise the ease that Sentry Guns can be damaged by indirect fire by explosive weapons.

Combat Mini-Sentry Gun

Due to their small size and quick build time, Mini-Sentry Guns can often be placed in positions that a normal Sentry Gun cannot and perform exceptionally well when laid in concealed areas. These advantages, coupled with the bonus health the Gunslinger provides, allow for relatively aggressive play. They only cost 100 metal to deploy, so you should be able to provide a constant source of suppressive gunfire. Don't hesitate to place them out in the open as distractions or to cover a retreat; players tend to fire at them before you. They are far less useful when placed in defensive positions, where their reduced firepower won't be able to fight enemies off.

Dispenser

A Dispenser is typically the first building you want to construct so that you have a stable source of metal. Placing Dispensers in forward locations will keep teammates healthy and allow them to hold an area for a long period of time. You can hop on your own Dispenser to climb to inaccessible places.

Teleporter

Always have a Teleporter system active to help a steady tide of reinforcements reach the front lines, even if the distance covered is short. Rotate the Exit so that it faces away from a wall and allows teammates to see the battlefield immediately upon coming through. Be aware that the Teleporter Exit's placement can impact the flow of battle; sending teammates to poor locations will weaken both your offense and defense. If either Teleporter member comes under fire, hitting one end with your melee weapon will repair the other. If guarding your buildings, refrain from standing on your Teleporter Exit, as enemy Spies are able to come through and telefrag you.