The sentinel comes in two flavours. The regular sentinel fires lasers and comes with a base-themed skin. To switch to a sentinel which shoots nails and has a medieval-style skin, set spawnflag 4 on the monster.
The sentinel was inspired partly by the spikemines from Scourge Of Armagon, partly by the spike-shooters in e2m7, and also by the desire to have another aerial enemy more Quakey than bob.
The result is more akin to a trap than a conventional monster; something that is deployed under special circumstances and behaves systematically rather than intelligently.
The best placement for sentinels is in chambers that are high, wide, or both. The more space the better because it is the attempt by the player to predict and avoid the unrelenting projectile barrage that defines sentinel combat, and this requires time.
The eerie way in which the sentinel will closely match the player’s height, but remain otherwise unresponsive to events is effective if you can herd the player in a particular direction. Have the player tracked by sentinels while trying to ascend a series of ladders and catwalks in a giant hangar. Deploy them in the sky over an outdoor landscape across which the player must navigate to safety like a 3D minefield.
Any weapon with a good range is fine for knocking sentinels out of the air – their relatively slow movement means the player only has to correctly time their emergence from cover.