You also need some dragon scales in order to turn Amdusias human again, which can be found on the ground along the path to the blood alter. Read the book in your inventory that the librarian gave to you (a book of counter spells) and it will turn Amdusias human (if you have the blood and scales).
If you have already picked your battle tower servants, then you are past that point and on Sentinel Island. Have you explored everywhere on the island?
You got a book of magic that can control wind when you rescued the guy on the top of the pillar (near the trainer Kenneth). There is a windy spot near where you start on the island with a tombstone and three candles...
Standing in the windy area, read the book in your inventory to stop the wind, light the 3 candles, talk to Sassan's ghost, go fetch a ring for her to complete a resurrection ceremony, then follow her running to the battle tower to get the force field there dropped.
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