If you're thinking of the physical space:
* Palace: A luxurious and grand dwelling, the opposite of a dark and grim dungeon.
* Mansion: A large, impressive house.
* Tower: While not always luxurious, a tower implies a more positive, perhaps even romanticized image, unlike a dungeon.
* Sanctuary: A safe and sacred place, a complete opposite of a dungeon's confinement and danger.
* Garden: A pleasant and peaceful outdoor space.
If you're thinking of the imprisonment aspect:
* Freedom: The ability to move and act without restriction.
* Liberty: The state of being free from oppression or control.
* Release: The act of setting someone free from confinement.
* Emancipation: The act of setting free from slavery or oppression.
If you're thinking of the negative connotations of darkness, misery, and danger:
* Paradise: A perfect or idyllic place.
* Heaven: A state of perfect happiness and peace.
* Elysium: A place of great happiness and peace.
* Utopia: An ideal society.
Ultimately, the best antonym for "dungeon" will depend on the specific context in which you are using it.