After entering the archway the path you take is simple enough, just walk straightforward to the door which leads down to the tombs. Though be prepared for a fight, as the Guardian will block the Princess and her comrades path and approach once the Princess gains sight of it.

Note: The Guardian is a hulking skeletal golem, although it doesn't have the golem template and therefore much easier to take down if the Princess makes use of sneak attacks with a bow, while Christano and Myra engage it in melee. This strategy however can be considered cheating through exploiting a bug.
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