Engineer
The Engineer is considered an offclass in 6v6 and is run in Highlander. An invaluable defense tool, the Engineer's sentries, dispensers, and teleporters are very helpful to defend a point or cart.
In Sixes
Main Article: Engineer (6v6)
In 6v6, the Engineer is an offclass. Most often used on the map cp_gravelpit, the Engineer is only used to defend last on most maps. The Engineer's sentry is key to getting picks on unsuspecting classes and creating number advantages. It almost always requires either a Demoman or an uber to destroy, making it a great stalling tool as well.
In Highlander
Main Article: Engineer (Highlander)
The Engineer's main job in Highlander is to defend points and other positions. Through the use of the sentry, the Engineer is easily able to defend and stall for a point in time. Highlander presents a new risk, however, in the Spy, whose Sapper can destroy the Engineer's buildings. The Engineer can also use such tools as The Wrangler and The Gunslinger to be more effective on offense or to assist in defense.