Cityscape Fighting (Apocalypse)
A brief summary of actions within Xcom Apocalypse:
Infiltration
A UFO will hover over a building and attempt to insert aliens via a 'grey smokey tube'. If an X-COM Base is hidden in the basement of that building, a Base Defense mission will immediately commence.
Alien Search
A UFO will hover over a building and drop blue particles with the sole purpose of scanning the building for an X-Com Base. This is what is typically called "Micronoid Rain". X-Com may intiate a search by landing a craft with troops at any building and selecting "Search". If an alien presence is detected, a battlescape mission will commence. If no aliens, the organisation will file a protest by becoming very slightly more hostile.
Alien Bombing
The aliens will target a building with their craft and attack it with the purpose of destroying it. If the building is above a hidden X-Com base and when the topside structure sustains massive damage and collapse, it can destroy the base underneath. That is: if a great portion of the building is destroyed or if the personel/road vehicle building piece is destroyed, the X-Com base is destroyed also.
Illegal Flyer
Any organisation (including X-Com) can attack any building with vehicles. An alert is generated (if enabled) when other organisations send out craft, hence, "Illegal Fliers detected". Xcom intervention is not warrented unless the building is above your base. Score is not affected unless Xcom is involved.
Raids
Organizations unfriendly or outright hostile to each another will conduct actions as listed in the Message Bar as either "raided by" or "stormed by" or "attacked by". This has no relevance to X-Com unless the target building contains an Xcom base, which a base defence mission will occur. X-Com funds will be lowered/stolen/decreased due to the presence of invaders and to damage sustained within the base. X-Com may intiate a raid, as well, by landing a craft with troops at any building and selecting "Raid".
Overspawn Drop
The aliens focus more on destruction instead of infiltration. Motherships will drop a single Overspawn each onto Mega-Primus with the intent of massive damage and destruction of its buildings, roads and infrastructure.
Apocalypse Mission
When X-COM destroys the Control Chamber in the Alien Dimension, it eliminates the Alien's ability to grow new UFOs. The alien horde now realise their imminent defeat... with a final act of desperation by sending the remnents of their fleet to Mega-Primus. All remaining UFOs in the alien dimension (except probes) will swarm into Mega-Primus on the next scheduled mission, to destroy the whole city. With the critically important Control Chamber destroyed and no more UFOs to infiltrate Mega Primus, the alien's ultimate objective of sustaining the Micronoid lifeforms with human hosts has essentially failed.