Mangar's Tower resides in the south-west corner of Skara Brae, where Night Archer and Tempest streets collide. It's known to have at least 5 floors of precarious and evil things that only beckon those who wish death upon themselves.
The gate to the tower is locked tight and is impossible to pass without an onyx key. But there is a secret passage some have discovered hidden deep beneath the sewers that can lead to another manner of access.
The evil wizard Mangar the Dark resides therein, protected behind a wall imbued with impenetrable magic. The wall is etched with three shapes - a square, a circle, and a triangle. It's reported1 that all 3 of those shapes must be possessed to complete the key needed to bypass this barrier. Two of those shapes are scattered elsewhere in the city of Skara Brae - hidden in Harkyn's Castle and Kylearan's Tower.
When Tarjan destroyed Skara Brae, the tower became inaccessible, its fate left in the hands of time. With Mangar's defeat, the tower is unlikely to have remained intact.
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