Some can cloak.

Corridor 7 (a.k.a. Corridor 7 - Alien Invasion) is a first-person shooter developed and published by Capstone Software. It was later re-released by Gametek, Expert Software and Ziggurat Interactive. This video shows footage from the disk version of the game. The CD-Rom version contains more enemies, weapons and levels.

The story is set in 2012, and someone has beaten Elon Musk for decades in sending a spaceship to Mars. The ship returned with a small object of unknown origin. It was taken for research to a special underground research facility and secured in its lowest labs, down "Corridor 7". When the object got exposed to gamma radiation, it opened a portal to alien world, and its inhabitants began coming through the portal in order to invade Earth. You're an unnamed Special forces agent who gets sent down to Corridor 7 and deal with the problem.

Corridor 7 uses an enhanced version of the Wolfenstein 3D engine. Hence it also has no different heights, maps are built on rectangular grids and ceiling and floor are not textured. Unlike most classical shooters, the goal is not reaching the exit. Instead you have to kill all enemies in the level. You an elevator to get to each level, and once you have killed all enemies, you can use the same elevator to go down to the next level.
Not all items are lying on the floor, medpaks and ammo are usually received from dispensers in the wall. There are no keycards, but you get access to certain doors when using certain computer terminals which can be found in the level. There is also a healing chamber in each level which restores your health, but has also finite resources. You also have heat- and nightvision which can charge at certain stations.