Siege Turtles are heavy artillery units that can be encountered with Luxon forces in several areas. They move fairly slowly, and will periodically fire a mounted cannon that has a very long range and strikes all enemies in the area of attack with high amounts of damage.
In PvP combat, such as in Fort Aspenwood, they are primary targets for the opposing team. However, the turtles have a strong defense against direct damage in the skill Turtle Shell, so as long as they are healed periodically they are not very vulnerable (unless all the enemies focus on the turtle). This skill however leaves them vulnerable to health degeneration attacks such as hexes and conditions.
Kurzick Juggernaut units seem to be designed specifically to fight Siege Turtles and will flip over a Siege Turtle (for 5 whole seconds) disabling its cannon attacks.
In the Gyala Hatchery mission, the Siege Turtles require a player character to mark strike zones with a Smoke Canister to fire.
Their siege attacks are classed as skills, and therefore can only be stopped by interrupts that target skills. Siege Attacks can not be disabled with Diversion.