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. The Engineer and his buildings will frequently be targeted by Demomen and Spies; a well-timed enemy ÜberCharge 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 to defend them.

Primary Weapons

Shotgun

The Shotgun is a deceptively powerful weapon at close to medium range. Use it to defend yourself and scare off enemies that your Sentry Gun cannot attack. If a Spy starts to sap your buildings, use the Shotgun to kill him before removing the Sappers. 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. Equip the Pistol so that you can conserve your Revenge crits. You only have three shots, so aim carefully. If you are the one that destroys your own Sentry Gun, you will still receive Revenge crits.

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

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. If you attack an enemy Spy next to a friendly structure, you will hit the building instead of him. Use your Shotgun 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 occasional Revenge crits you'll 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.

Combat Mini-Sentry Gun

Mini-Sentry Guns are cheap to deploy, so don't hesitate to place them out in the open as distractions; 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. 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, even if the distance covered is short. 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. Use Teleporters to help a steady tide of reinforcements reach the front lines. Hitting a built entrance/exit while the other is building will fully repair the constructing end.