There are many iterations of Autobot City in the Multiverse. Most can be considered NPC facilities, however. There are currently two that are considered important "in play" at present.
The first is Autobot City proper, which is located on the Earth that corresponds with Transformers-1. It is a massive, Transformer-sized city-fortress, capable of withstanding considerable bombardment and even a full-scale attack. It serves as Autobot headquarters on that planet, and is supplemented by the wreckage of the Ark, which served as Autobot headquarters until the construction of Autobot City.
The second is Metroplex. Metroplex is a Transformer roughly the size of a city which can be joined with Autobot City to form an even larger city-fortress, but is currently stationed in New Washginton. Metroplex is fully sentient and acts as Autobot headquarters for most of the Union. When the situation calls for it - which is rare - Metroplex can transform into a mobile battle station, or in even more dire situations a robot that is nearly three miles tall that can literally shrug off nuclear explosions. (Metroplex is an NPC which cannot be applied for).
This article is a bit of a stub and will (hopefully) be expanded on later.
