The secret level known as the shadow tower or shadow dungeon holds a fantastic bounty of weapons which are unavailable in the normal game.

To get there you will need a dark guide post. These are available in 2 places.
1)One is available in a chest located in rieubaine castle.
2)They can be obtained by killing palmira bees.

The latter method is the better as you can carry up to 9 dark guide posts at a time. (You can only use a dark guide post once.)

To obtain a dark guide post, you must find a palmira bee by hitting elements of the landscape with your weapon or a palmira action (lightning storm works best). The best places to attack are the blue pillar at saramad hill (next to the fountain), anywhere where you can find billiana tree roots (i.e. the underground shrine or the abandoned road) and the small rod-like pillars at the entrance of the soaring tower.

To kill a palmira bee, face with your back towards a known source of palmira bees, use a surrounding palmira action (one that attacks all sides) and then use the same action once the bee appears - thus killing it!

Next how to use a dark guide post - simply go to a save crystal, select 'status' and then go into items and use a dark guide post!
You will be prompted to save (recommended!)

For further help on the hidden dungeon/shadow tower try posting on the neoseeker evergrace forum!