Tourian is the main base of the Space Pirates operations on Zebes and is the final area of Metroid, Super Metroid and Metroid: Zero Mission. Tourain is located in the heart of Zebes and is accessible from Crateria, the landing zone.
In the cave that houses the entrance to Tourain are 4 statues (in Super) or 2 (Zero mission and original) these are Kraid, Phantoon, Draygon and Ridley. Each of these must be defeated to gain entry to the base.
In all three games, Tourian is the only area in which Metroids are faced. Also, the Rinkas can be frozen with the Ice Beam; once frozen the turret that spawned that Rinka won't spawn a second one until it is destroyed or flies offscreen.
Also in all three games, in the final room, four Zebetites must be destroyed, and they can only be destroyed with a constant barrage of Missiles.
In the original game, Tourain is only accessible when you defeat Kraid and Ridley. Once these two are dead, their respective statutes in the statue room will crumble, thus allowing you to proceed.
To defeat the Metroids, you must freeze them with the Ice Beam, then blast them blast them with five missiles. If a Metroid latches onto Samus, the only defense to get it off her is to drop into Morph Ball and detonate Bombs.
Also unique to Tourain are Rinkas, turrets built into the walls which fire energy blasts and gun turrets, small turrets which shoot energy bullets.
The battle against Mother Brain herself consists of two parts, destroying her glass container then destroying her. Unlike Super Metroid she is totally immobile and relies on Rinkas and gun turrets to attack Samus. Once she is destroyed a count down timer activates and Samus must escape via a vertical shaft before Tourian explodes.
 Super Metroid
Tourian reappears in Super Metroid with two areas, "Old Tourian" which is the original, abandoned and derelict one that is found in Metroid 1, now revealed to have been located in Crateria, very close to Samus's Gunship; and the "New Tourain" which is located next to Old Tourain, but this time fully operational.
Like the original game, Tourain requires you to defeat all 4 bosses which consists of Kraid, Phantoon, Draygon and finally Ridley. Defeating these four crumbles their statues in the statue room and allows Samus to proceed onwards.
The layout is quite similar to the original Tourain with the infestation of Metroids, once again defeatable by using the Ice Beam to freeze them, then five Missiles or a Super Missile. Power Bombs can also be used to defeat the Metroids.
Unique to this Tourain, however, is the feeding room/under construction room for the "Baby Metroid" with many enemies located there mysteriously fossilized, and they are reduced to dust just by being touched. Even the fearsome Torizo couldn't escape this fate. A lone Side Hopper hops towards Samus but is instantly drained of its energy by the Baby Metroid, now ginormous in size and reduces it to dust like the others. The Metroid proceeds to attack Samus but at the last moment stops and remembers who she is.
As Samus proceeds she meets up with a reincarnated Mother Brain once again relying on the attacks of Rinkas and Gun turrets. As Samus "destroys" the brain, Mother Brain awakes to revel a huge, mobile body hidden underneath. She has many attacks including the powerful laser drain attack which almost kills Samus until the intervention of the Baby Metroid, who saps the life out of Mother Brain and rejuvenates Samus with the stolen energy.
Mother brain awakes and proceeds to kill the baby Metroid, in doing so spilling some Metroid DNA onto Samus's Power Suit, and in the process Samus gained the Hyper Beam. This beam obliterates Mother Brain, which once again sets of the self destruct alarm. Samus escapes through an area heavy with Space Pirates and escapes though a hatch leading to the long shaft in old Tourian. Samus escapes Zebes, with the explosion blowing the whole planet up along with Mother Brain and Tourain, for good.
 Metroid: Zero Mission
Tourian's setup is very similar to how it was set up in the original Metroid, and the remains of deceased Space Pirates can be seen in the background. The Metroids also jump out of the background, and once frozen either 5 Missiles or a Super Missile will dispatch it.
Unique to this Tourian setup is the fight against Mother Brain. After shattering the glass with Missiles or Super Missiles, Mother Brain is only vulnerable when her eye is open, which sometimes shoots a laser upon opening! Jump to dodge this then retaliate with Super Missiles or a Charge Beam shot!
The escape run is longer than it was in the original Metroid as you still have to run back to the ship and take off.