This is a wonderful little adventure - just the perfect dungeon delve. My favorite characteristic of these adventures is that they are exactly the right size for a busy DM to read once or twice and then run with ease and fun, with perfect pacing and enough interest to build a world with the players' help (as they always contribute more than they realize ;-).
@Katherine M. -- I interpreted the map that Chariz gives the characters as a map of the general area (allowing them to more easily locate the circle of stones) not a map of the dungeon. But, I loved your dungeon map so much, I placed it on the fourth skeleton in the first room: an adventurer from ages past who'd been subdued by the undead skeletons and laid to rest on the fourth platform. My players found it with great delight as I gave them a rolled and wrinkled print out. Thank you for that!
@Paul L -- As for Chariz's motivation for not searching out the Mystic Circle himself, this is how I approached it:
Chariz is a studious and busy wizard, prefering the research he conducts in his tower to wandering the woods; when he can, he always employs others to do the digging for artifacts he's learned about from his studies. On the day he asks the players to seek out the Mystic Circle, a tired and bleary-eyed Chariz is watching over numerous projects that that require his full attention for many days (e.g. potions, scroll writing, perhaps some scrying, etc). In his younger years when he explored more, he kept an eye out for the rumored circle when his work took him into the woods, but he never found it. In the past month he's uncovered evidence in newly aquired tomes that leads him to believe the circle and the dungeon below it is real, and his interest is piqued by the silver disk described in his research. This is his motivation for hiring the players to take on a more earnest search for the circle.