Metroid is the home base of Mother Brain and her minions in both the cartoon and comic book continuities. It is an asteroid/planet that seems to be at least somewhat artifical, with underground corridors that are packed with deadly foes and technology.
In cartoon continuity, amongst the technology Mother Brain has at her disposal is the all-knowing mind mirror, through which she can apparently see anything in Videoland (and possibly beyond). She can also ask the mirror for advice, though it frequently responds to her queries with insults.
The N Team sometimes visited the world in an effort to put an end to Mother Brain's schemes in Season 1 and the Season 2 premiere. At the end of the Season 2 premiere ("Gameboy"), the world had a power overload which would lead to the assumption it would be destroyed, as Mother Brain and cronies had to evacuate. While Metroid continues to appear beyond that point, the world is seen far less, and the N Team never goes there again.
Cartoon
Metroid is a metallic, brain-shaped asteroid or spacecraft. It sometimes emits gas or steam. It is primarily metallic, though there are plenty of natural components in the deadly corridors, which are filled with Mother Brain's minions, platforms, and liquid.
Mother Brain's lair is accessible from the corridors and vice-versa. It has a brown metallic interior. There is a viewscreen on which Mother Brain can watch events seemingly anywhere in Videoland. The lair is also lined with warps to other video worlds.
There is a tower in which Mother Brain can hold prisoners. (Kevin In Videoland)