Gritnea Tower is the location of Chapter 28 in Fire Emblem: Path of Radiance. Chapter 28: Twisted Tower is the second last chapter in Path of Radiance and considered rather easy when compared to Chapter 27: A Moment of Fate and Path of Radiance's Endgame.
Gritnea Tower is located just outside of the Crimean capital, Melior. It's purpose before The Mad King's War is unknown, although it is very possible that it may have been a Crimean prison or even a research facility before Ashnard invaded Crimea.
Gritnea Tower was the building used by Ashnard to do experiments on captured laguz that he considered "simple tools". Izuka was the overseer of the experiments preformed at the tower. Izuka is the man who has been commonly accepted as the creator of the Feral drug, which he developed in the Gritnea Tower. The Feral drug distorted the bodies and minds of the laguz it was given to and made them warped, insane fighting machines. Once given the drug the laguz could not return to humanoid form and they went mad killing anything they could, although Izuka had them trained not to attack Daein soldiers. Some of the Laguz were stored in a pitch-black basement room where they were made to fight Daein soldiers that wished to prove their worth.
In Path Of Radiance, Gritnea Tower is where Leanne is held prisoner after the Black Knight captures her and presents her to Ashnard. In Chapter 28: Twisted Tower, Ike and Tibarn lead a small force to capture the tower and free Leanne. The forces defending the tower are mainly Feral Laguz that Izuka released to aide the few soldiers. After winning the battle Ike and company learn of the true purpose the Gritnea Tower served.