The FA20HL is a surface based stationary anti-aircraft weapon. Designed by Melies, it is a powerful dual lasercannon with high power; a half a dozen of these units could easily take out a fully armed NEXT. They are used by Leonemeccanica in order to protect their energy recycling plant, Megalis. They would be put in continued used after Line Ark took over the facility.
It is possible that the turret's name is based off of two brothers that appeared in Armored Core 2, Castor and Pollux.