"We've Got Hostiles!"
Much of the building here is made of concrete walls. Green military supply caches are stored around, along with the same wooden boxes found earlier. Chain link fences block certain areas where visibility is still important, but visibility only.
Consider placing red fire doors and the boxes that activate them in between areas of interest, especially in range of tripmines. They provide a convenient way to prevent backtracking or restricting it to air ducts.
As per the introduction, the military is moving into the facility, killing anyone associated with the experiment. Their influence on the facility should be obvious here. Tripmines make accessing certain parts of the facility more difficult. Turrets are also commonplace, as are their activation tripwires. Explosive barrels make good decoration, and you can place them near groups of enemies or turret tripwire to make clearing out an area more or less difficult.
Teleporting Vortigaunts are common, and a good mapping tactic is to have them teleport near tripwires so players have to be quick in silencing them. Scientists and security can be found, but try to hold them for the safer parts of the chapter.
Obviously, the military make their debut here. A good idea is to place them in squads, with multiple MP5 wielders, one or two that can make use of the attached grenade launcher, and maybe even some shotgun soldiers. Try to shoot for four in a squad at the max. (Note that all soldiers can make use of frag grenades to flush a hidden player out)
<gallery> Image:C1a3 confrontation.jpg|Note the nondescript concrete walls and the red blast door and tripmine on the left Image:C1a3 turrets.jpg|Turrets are common in the early part of the chapter, as are explosive barrels