The Tower at the Edge of the World — Victoria Goddard

East of the Sun ... West of Astandalas ... As far away from anywhere as it is possible to be and still be civilized, there is tower. To its east is an endless abyss; to the north and south are grassy dunes haunted only by birds; and to the west is the sea. Far on the other side of the silver horizon is the Empire of Astandalas, which holds together five worlds in a net of magic. It has one inhabitant. Exiled to the tower at sixteen, a nameless young man is content to perform his rituals, read his books of poetry, and fulfil his destiny tending to the magic that anchors the Empire. He has been very carefully trained not to wonder about anything else. One day he finds a key.