In Highlander, the Engineer is in charge of locking down areas for his team and making sure all his teammates are getting to the fight as quickly as possible with as much health and ammo as possible. Using both regular and mini-sentries, he can prevent the other team from utilizing entire regions of a map or force them to waste time attacking him while his team retaliates. Using teleporters and dispensers, he can support his team and keep them fighting until the enemy is defeated.
In most cases, offensive engineering requires use of the Gunslinger. The Engineer on offense is responsible for keeping the team's flank clear and for making sure his fellow teammates on the flank are healthy and stocked with ammo. Efficient and frequent use of sentries and dispensers is vital for preventing an enemy Soldier or Scout from getting behind the team and wreaking havoc. The best Engineers pair proper building management with strong deathmatch skill to fortify any area they enter with their teammates and make sure that once ground is won, it is not lost.
During payload offense, one of the Engineer's primary goals is to push the payload cart. Most teams designate their Engineer and their Scout to cart pushing, maximising push time at x3. Engineer's should try to keep teleporters up as well, as the walk from respawn to the cart can sometimes be very long. Engineer's should also try to keep minisentries down at all times to increase damage output or to cover a flank. Weapons used by the Engineer on offense, whether in 5CP, Payload or King of the Hill, vary and are often up to the engineer. However, most of the time, the stock shotgun is a strong choice on offense. The Short Circuit, Wrangler and Pistol are usually selected according to the map type and objective. For example, a lot of Engineers run the Short Circuit on Payload offense to block spam while pushing the cart, which provides them with metal. On the other hand, a lot of Engineers use the Wrangler or the Pistol on 5CP / KOTH maps, where extra damage and firepower is necessary.
Spychecking is a team's job and responsibility, but at the same time, an engineer is one of the worst enemies of the spies. Other classes may be too busy / too many things on their plate to keep a constant vigilance for spies. For example, a medic has to keep track of Uber advantage and positioning, a heavy has to call, etc. However, an engineer is not burdened with so many responsibilities such that he can still look out for spies while doing his job as described above. An engineer's shotgun and sentries are potent weapons, with the ability to decimate spies within a few seconds. Often, a few rounds from one's shotgun and / or wrangled sentry are enough to kill the spy, or at least reveal him for your teammates to kill. In lower levels of competitive play, spies will often sap your buildings before going for picks, that can alert you and your team to him before he goes for a damaging pick. In higher levels of competitive play, spies will not give themselves away by doing that, and hence, spychecking is the only way to go.
While on defense, however, you are the prime target for spies. As the only person who can keep your sentry up, spies will be gunning for you. However, unlike on offense, you will have a lot of things to worry about, such as enemy ubers, spam, snipers, whether it is time to fall back, etc. Spies' saps can be deadly especially when coordinated with an uber or constant spam, as it is a guarantee that your sentry will go down. Often, good spies will communicate with their team to distract you for that one moment during which you turn your back and present an easy target. The most prudent way to counter this is to constantly communicate with your teammates, about the whereabouts of the enemy spies, etc. Moreover, it is imperative that as the engineer, you check your back / other hiding spots often whenever you can. One major mistake a lot of lower level engineers do is to sit still and hit one's sentry in order to upgrade/ repair it without moving. This opens the engineer up to attacks by spies. The way to counter this is to constantly move. The engineer's melee weapons do not register their hits instantaneously like a hitscan weapon; there is a slight delay. Hence, if the engineer times it right, he can move about or quickly turn around to check if there is a spy before turning back to hit his sentry.
There are few better defensive powerhouses than a Level 3 sentry in a strong position. Positioning, timing, and build order are important concepts for an engineer to understand on defense. Building too far forward or waiting too long can cost the team the relevant control point or even the whole round. It is the Engineer's job to tank damage from ubered Heavies and Demos so that his Medic doesn't need to pop early or fall under the pressure of nine attacking players. Kills are less important to an Engineer than merely surviving because most of the other team can't come within range of his sentry as long as it is active, and going down with his sentry can be costly if a domino effect leads to multiple captures before he can build again. The team leans on its Engineer to provide a safe fallback zone when they get lit and the other team is pushing forward. Engineer has a diverse array of tools available to him allowing for many different playstyles. The Eureka Effect and Rescue Ranger both give Engineer new advantages and playstyles but can also hinder his core mechanics (quickly build his sentry or defend himself with a shotgun). Therefore in the true spirit of the class, experimentation and innovation can sometimes reap his team huge benefits and throw the enemy team off their gameplan. Deathmatch-proficient engineers are sometimes known for running Gunslinger minisentries on Defense. This is something an Engineer should practice and discuss with his team before implementing in a match, as many teams depend on a larger sentry to tank ubers, and using the Gunslinger pushes the Engineer away from the combo towards the flank where he can't help against an Uber. However it is a noteworthy option on last ditch defenses when there is no time to build up another level 2/3 sentry. At the end of the day, the main objective for a engineer is to keep himself and his sentry gun alive, this once again requires good communication with your team and survivability on the engineer's part to know when to fall back and when to hold your ground.
Stopwatch gamemodes (Attack/Defend and Payload) give the defending players a setup time to get in position and prepare for the attack. This time is primarily utilized by the Engineer to build as many buildings as possible in the first position that needs to be defended. It varies from map to map, but in general the Engineer follows a basic rollout. First, he builds a teleporter entrance just outside spawn. Then, he runs to his first sentry position, grabs any ammo pack nearby, and builds a dispenser (it is important to have the building first to ensure a ready and constant stream of metal once the round starts. At this point it is common for teammates of the Engineer (usually a Pyro, Scout, and/or Soldier) to kill themselves near the Engineer. This allows him to pick up the metal from their dropped weapons to speed up the process of building. This metal is used immediately to build and begin upgrading the sentry. Once the sentry is at level 3, any remaining setup time is usually devoted to upgrading the dispenser and building a teleporter exit if there's time. Most standard maps allow for a "complete" rollout that results in a level 1 teleporter, a level 3 sentry, and a level 3 dispenser before time runs out. In some stopwatch maps, the BLU team gets a tiny niche where buildings are buildable inside spawn, (e.g., cp_gorge). Engineers on BLU can use setup time to build their own level 3 sentries, allowing for interesting tactics that usually involve sneaking the sentry into a position of ambush to kill retreating RED combo members.
On 5CP maps, some engineers build a teleporter entrance right outside spawn and run straight to mid (stopping at an ammo pack along the way for metal). At mid, the engineer should place his minisentry to provide fire support during the midfight. Either supporting with shotgun fire or wrangling it to ward off jumpers are viable strategies. After the midfight and during stalemates, the engineer is usually preoccupied with setting up his teleporter exit, dispenser and moving his minisentry to cover the flank. The problem to this approach is that the teleporter entrance is useless if the team wins mid. Many engineers omit the teleporter and just use all their metal for minisentries at mid. This can hurt the team by moving engineer from a supportive role to a combat one, but at the midfight, combat is all that really matters. After the fight, teleporters and dispensers can be placed without losing too much time.