It was built specifically to keep the minotaurMinotaur in.
If you are familiar with the story of the minotaurMinotaur, it was the son of Pasiphae, who was the king of crete'sCrete's (King Minos) wife. Minos wanted a sign from the gods that he was meant to rule, so Poseidon sent a great bull out of the sea, and told minosMinos to sacrifice it to him. Minos liked the bull so much that he didn't want to sacrifice it, so poseidonPoseidon took revenge by making Pasiphae fall in love with the bull. The bull in question is mentioned in a few other myths, and iI believe it went crazy, and was killed by herculesHercules as one of his labours, but don't quote me on that. This means that the Minotaur is not only the son of the prize bull, but also royalty. He couldn't simply lock up something that had:
- been sent as punishment from the gods
- was royalty.
He only later used the labyrinth to feed prisoners to the Minotaur.
Hope this helped :)