Alien Dimension (Apocalypse)

Alien Dimension is home to myriad of alien creatures in 1997 game X-Com: Apocalypse. It can be reached by traveling through dimensional gates.

Description
Ground in the alien world seems to consist mostly of mineral formations with crystalline areas dotted here and there. It's possible there is no other organic material in this particular place of alien dimension, apart from the bulging and writhing, enormous organic city. Landscape is as horrid as it is bleak to the extreme.

Airspace is filled with alien craft that consist of living tissue just like everything manufactured by the aliens.

Dimensional Gates
Three pyramid shaped anomalies known as dimensional gates float in the sky alien world. The pyramids are dimensional rifts that allow aliens to traverse into our dimension, where equal number of gates are being kept open by alien technology. Trying to enter one such gate without proper equipment results in utter anti-matter annihilation.

It is suggested that aliens have learned to transform the physics of their dimension to mimic those of other dimensions. Antimatter seems to be the medium that can be used to breach into alternate dimensions.

Once dimension gate generators from alien world are destroyed, the dimension starts to revert back to it's own regular form which is hostile towards matter from our world. Without aliens "terraforming" their dimension, existence there is impossible in the way humans experience it.

Mega-Primus War
Micronoids are hinted as being an extradimensional parasite that isn't native even to alien dimension, and the power behind this alien invasion. Considering how arid the dimension seems, it might be that micronoids had all but emptied alien dimension of whatever passes as energy there, and needed to find another reality to exist.

This eventually prompts the Mega-Primus war, though initially the micronoids attempts to infiltrate and subverse the Earth's last city via mind-controlled humans to prepare it's new host. When this scheme doesn't succeed as smoothly as planned, micronoids starts to use gradually more and more aggressive methods.

As the X-Com organisation breaches into alien dimension and leads an assault towards their city, an all-out war ensues.

Organic City
Entire alien city is an enclosed living organism. Ten large protrusions are connected by a tract that resembles an umbilical cord. The protrusions act as different facilities and have various functions that keeps the alien brood alive.
 * Sleeping Chamber that rejuvenates smaller alien species has a single point of entry to the gigantic structure. After disabling sleeping chamber access to the umbilical cord-like tract can be used to enter different alien buildings. However, one particular building is always connected to two other buildings, so advancing inside the city has to proceed in a linear fashion.
 * Food Chamber provides nutrition, and after it's destruction opens up an access to Alien Farm, which is continuously terraforming atmospheric conditions of the alien world. As a curiosity, Sectoids can be encountered in Food Chamber, which either means that sectoids found a way to hop into different dimension and as a sorry consequence gave a tip for micronoids for a new assault, or that micronoids somehow managed to abduct the abductees.
 * Maintenance Chamber is essentially the heart of the city: it pumps nutrients to all the other larger organs of the alien complex.
 * Incubation Chamber is where all new aliens are grown. Most Anthropods, Skeletoids and Psimorph are hatched here, as well as the lesser creatures such as Hyperworm and Multiworm that are essential for alien reproduction.
 * Control Chamber is the brain organ of the city. Without brains guiding various entities and giving orders via nerve impulses, the city cannot function in an organized fashion.
 * Spawning Chamber houses the Queenspawn, an alien creature responsible for all the alien eggs.
 * Organic Factory manufactures all organic armaments used by alien troops. It is likely that megaspawns are bioconstructed here. Factory also matures alien ships from small embryos that develop into mushroom looking growths. Once fully formed, the alien ship detaches from factory organ.
 * Megapod Chamber develops enormous seeds that, once planted, may grow into new buildings.
 * Dimension Gate Generator is the building that keeps connection between dimensions sustainable. Destroying it is going to sever any link between alien world and Earth.

Alien Life Forms

 * Brainsucker
 * Popper
 * Spitter
 * Hyperworm
 * Multiworm
 * Anthropod
 * Skeletoid
 * Psimorph
 * Megaspawn
 * Overspawn
 * Queenspawn
 * Micronoids
 * Alien Ships