It is in Senja, about 101 steps north of the spot the map marks with that name.

And 3 down from the surface once you are there.

• 1 — Senja, as the map itself names it
• 2 — the ground above the treasure

• Key 4502: a locked door bars the way, and that is the number of the key that opens it.
• Something for beholder, minotaur guard and slime — they live down there.

• 1 — the chest
• 2 — the chest
• 3 — Senja Castle Exit

The strongest 14 of 17 kinds whose nests reach this place.

There are 3 chests here under 3 quest numbers, so you can take them all.