Marines Red Alert is a moderate long mappack that takes approximately more than 45 minutes and is composed of 4 maps with multiple choices for you to take, they are not linear and you can complete them with more exploration. It means that these maps can be completed also quite fast if you simply want to avoid getting to additional places like outdoor areas, optional rooms, secrets etc. - it's all up to you how do you want to complete them. The mappack takes place on Mars, in the UAC colony where our lone Space Marine is surprised by a demon horde attack in the middle of doing something, the start is quite similar to the Extermination Day ;-). WAD has a full narration with using the DOOM 3/Half-Life voiceovers all over the UAC facility on Mars, remember that this is just a human colony with living quarters, mining facilities, storage, everything. You might also see the development of the demonic invasion the more you play, with first level all you will see are just the slain bodies of fellow Marines or UAC personel, pretty much anywhere. With later maps that you are going to explore you will notice that something is really off, strange demonic markings are placed, there are bodies hanging from the ceiling and some demons made their own lairs. Rarely you will encounter the trapped Marines that might help you in taking down the demons, but their AI is pretty basic because it's the custom added Marine, they also die very quickly because of this. Mappack has also custom monsters that you are going to encounter, most of them are basic scientists or former security personel, however some of them are deadly like the UAC Spec Ops that simply turn invisible in darkness. Because the levels are pretty dark it's good to have a flashlight addon installed because of this, plus you might have problems with Save/Loads, they might not work with gameplay mods and addons.