It's an npc_turret_ground.
"In the Nexus building (the
d3_c17_10b map) these were seen entrenched within the floor, and could
only be killed by blast damage, like lobbing a hand grenade inside its
lift. That lift, and the ability to destroy the turret, is not part of
this entity. (The lift was a func_door brush rig, and the player really
killed an invisible npc_bullseye, filtered to be killed by blast damage
only by a filter_damage_type, that signalled the turret to descend and
lock."
link
You
should be able to parent the turret to a simple door brush (using the
parent name). Never did it myself but the programmers at Valve are
pretty proud of this feature.
Use a trigger_multiple (that activates the func_door) near the env_beam to simulate the lasers.
This is all theory and pretty complex but should work.
