Clash of the Titans

In ancient times, after defeating their predecessors, the Titans, the gods divided the Universe among themselves. Zeus took the skies, Poseidon took the seas, and Hades was left with the Underworld upon being tricked by Zeus. The gods created the mortals, whose faith and prayers fueled the gods' immortality. As time passed, however, mortals began to question and soon resist their creators, angering the Olympians.
A fisherman named Spyros finds a coffin adrift in the sea, discovering a baby, Perseus and his mother DanaÃ«. Spyros and his wife, Marmara, raise Perseus as their own son, alongside their daughter, Tekla. One day, an adult Perseus and his family watch soldiers from the city of Argos destroying the statue of Zeus. Infuriated at this, the Gods unleash the Furies who attack the soldiers and destroy the family's fishing vessel. Only Perseus survives and is found by a group of the soldiers.
Perseus is brought before King Cepheus and Queen Cassiopeia, who are celebrating their campaign against the gods. Queen Cassiopeia compares her daughter Andromeda to the gods and boasts that she is more beautiful than Aphrodite. The revelry is cut short by the arrival of Hades, who has been given leave by Zeus to punish the mortals for their defiance. Hades threatens to unleash his monster, the Kraken, against Argos, unless Andromeda is offered as a sacrifice. Before leaving, he reveals that Perseus is a demigod and the son of Zeus. Perseus meets Io, who confirms his origin. According to Io, a king of Argos named Acrisius defied the Gods and laid siege on Olympus. Zeus wanted to teach him a lesson, but he could not kill the humans because he loved them so much. Instead, he decided to make an example out of Calibos and disguised himself as the king, and then made love to his wife. When Calibos realized this, he executed his wife and son, while cursing Zeus and denying him their bodies. Zeus punishes him further by deforming him. Though Queen Danae dies, her son, who turns out to be Perseus, survives.
Perseus leads the King's Guard to the Stygian Witches, looking for a way to kill the Kraken. After being betrayed by the power-hungry Hades, Zeus gives Perseus a sword forged on Mount Olympus and a winged horse named Pegasus. Perseus refuses both, but the captain of the King's guard, Draco, keeps the sword for when Perseus needs it. Soon after, they are attacked by Calibos, an agent of Hades. Calibos was once the king of Argos named Acrisius, thus he shared Zeus as his common enemy. As Hades gives him powers, he promises Hades that he will kill Perseus, who he knows as Zeus' son with his wife. Draco severs Calibos's hand and forces him to flee. The band give chase but are attacked by giant scorpions called Scorpioxs that spring from spilled drops of Calibos's blood. They are saved by a band of Djinn, non-human desert sorcerers led by Sheik Suleiman, who tame the remaining Scorpioxs. Also wishing for the gods' defeat, the Djinn lend their aid to Perseus and his band.
The group arrives at the lair of the Stygian Witches and learn that to kill the Kraken, they must obtain and use the head of Medusa, a gorgon who resides in a temple in the Underworld. Any living creature that looks on Medusa's eyes turns into stone. Perseus, Io, Suleiman, Draco, and their remaining men cross into the Underworld. The men enter Medusa's temple lair while Io, being a woman and forbidden from entering, remains outside. Medusa kills everyone but Perseus, who manages to behead her by using his reflective shield to see her with his back turned. As he leaves the temple, Calibos appears behind Io and fatally stabs her. As Perseus and Calibos fight, Perseus accepts that he is a son of Zeus, picks up the Olympian sword, and stabs Calibos through the chest, which restores him to his human form. With his last breath, Calibos urges Perseus not to become a God. Before dying, Io urges Perseus to leave her and save Andromeda and Argos. Pegasus takes Perseus back to Argos as the Kraken is released. The people of Argos seize and bind Andromeda to offer her to the Kraken. Meanwhile, Hades reveals he does not require the faith or worship of mortals as Zeus does, as he has learned to survive on their fear. Hades then subdues the weakened Zeus.
Perseus arrives at Argos and exposes Medusa's head to the Kraken, which makes eye contact just before it is able to reach Andromeda. The Kraken, petrified, slowly turns to stone and shatters. Prokopion, the insane leader of the Cult of Hades, tries to kill Perseus, but Cepheus stops him and is stabbed, before both are killed when the Kraken's petrified hand falls on them. Hades appears, intending to kill Perseus, but Perseus, calling upon Zeus, throws his sword at Hades, forcing him back to the Underworld. Perseus rescues Andromeda, who is now crowned Queen of Argos. She asks Perseus to stay by her side as King, but he declines. Perseus also refuses another offer of godhood from Zeus, who then proclaims that if Perseus is to live as a human he should not be alone and subsequently revives Io.

Question: WHO IS FORBIDDEN FROM ENTERING THE TEMPLE?
Answer: IO.