I haven't reached that point in the game yet, but this is from a walkthrough by seorin. I can't vouch for the accuracy and I do not know the lock level of the chests.
-The ten piece Archmage set is hidden in treasure chests around the fortress.
1: The ring is straight northeast of the entrance, right next to the first
2: The cuirass is slightly northeast of the last (northernmost) generator.
3: The gauntlets are just a couple tents south of the cuirass.
4: The earrings are on the same platform as the gauntlets and cuirass, at its
5: The belt is on the same platform as the gauntlets, cuirass, and earrings,
but one level up. It's just south of the ramp leading up to the top level.
6: The leggings are on the same platform as the last several pieces, but at
the very top, on the northwest side.
7: The helmet is hidden on a tiny platform south of Stone's Greenery.
8: The necklace is on the east side of a small platform overlooking Stone's
Greenery from the north.
9: The bracelet is on the northeastern side of the lower tier of the large
platform beneath Stone's Greenery.
10: The warhammer is northwest of the entrance, two levels up.