From TF2 Wiki
Turtling is the player-given name (referring to a turtle retreating into its shell) for when the majority of a team stay behind their own lines during a match rather than attacking and strongly reinforce a specific crucial location. This is usually very near or inside either their intelligence room or main control point. This strategy is mostly employed by groups of Engineers who fortify a location with multiple Sentry guns. Turtling could be considered a method of camping. It is not to be confused with strong defense, where the team has a good balance of offensive and defensive classes.
Turtling is frowned upon by most players as it is considered to be a cheap strategy which makes it too difficult for the attacking team to capture, usually resulting in Sudden death. It is also discouraged during sudden death itself, as opposing Engineers can end up guarding their own Sentries and waiting for the enemy to come to them, rather than attacking the enemy and so prolonging the round.
How to Break Through a Turtle
The simplest way is to Uber a Pyro or several Pyros at once. If they can get close enough they can cause lots of havoc. One or more Demomen would be the next best thing, since they would not need to get into the sentries' line of fire. A Heavy would be less recommended, due to the high knockback and his slow speed.
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