Engineer (Highlander)
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 (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.
On Offense
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 causing 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.
On Defense
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.