The Dark Intruder — Marion Zimmer Bradley

Xanadu. Not the Xanadu of Coleridge's poem, but-to the half-forgotten space drifter who discovered the place thirty years ago-a reasonable facsimile. It was a cloistered nun of a city, hidden behind a wide skirt of the most impassable mountains on Mars. And the city was more impassable than the mountains. No human being had ever entered it-yet. They'd tried. Two expeditions, twelve years apart, had vanished without trace, without explanation other than the dusty notebook Andrew had unearthed, today, from the rotted shreds of a skeleton's clothing.