Metroid is the home base of Mother Brain and her minions in both the cartoon and comic book continuities of Captain N: The Game Master. It is an asteroid/planet that seems to be at least somewhat artificial, with underground corridors that are packed with deadly foes and technology. It appears to be based on Zebes.
Metroid is a 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 and is filled with warps to other video worlds.
Metroid is home to many pieces of advanced technology. Among them is the all-knowing mind mirror, a monitor through which Mother Brain 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. There is also a tower in which Mother Brain can hold prisoners. ("Kevin in Videoland")
Mother Brain once described Metroid as her "nerve control center" and said that anything that happened to it would also happen to her. Support for this was seen when she thrashed about in pain after Eggplant Wizard shot Metroid with his Atomic Spud Gun and Duke chewed on some of the cables in her lair. ("Metroid Sweet Metroid")
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 her 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.
In the comic series, Metroid is stated to be an asteroid instead of a planet. In a Nintendo Comics System story detailing Metroid, it was stated that Metroid is completely controlled my Mother Brain. In that same story, Metroid is described in ways that make it seem more optimized for combat than in the cartoon.