The enemies inside are level 26 and level 27 (and some level 29 enemies in Dragon Knight Saga). In the center area of the fortress you will find a level 27 summoner named Xanlosch and his creature named Isabelle who are part of the quest Risk Life for Limb given by the necromancer, Jonelath. (Only if you chose him in The Prophecy). Behind Xanlosch is a crystal which generates the anti dragon field. Hit it to blow it up.
Just outside the walled area you find Xanlosch in, there is a smaller walled area with a teleporter that takes you to where Stone is. After fighting Stone (level 27) and his buddies (level 26), he will run away into a teleporter before you can kill him.
Through the teleporter is an arena where first you will have to face four level 27 sub-bosses (a troll, a demon, a beholder, and one other) as well as many spawned level 26 enemies such as imps, etc. all at the same time.
After taking them out, then you can finally take Stone out. Also, up one of the ramps opposite the way you came in is a big chest (all the doors up these ramps are locked too and cannot be lockpicked). When you go back to Stone's Fortress Headquarters, there is another area where there is a control panel where a lot of walking statues spawn. Inserting 5 invoker jewels (four in the same room and one in the aforementioned treasure chest) stops the statues from spawning and drops a small chest.
- You might not want to take out the shield generator by Xanlosch from afar with a bow and then take Dragon Form immediately after dialogue to take out the two flying enemies Lord Wahat and Admiral Mirihim who appear before letting Xanlosch talk to you again. This caused his calling them out for aid after the protagonist killed them, and Xanlosch and his creature became non-hostile non-interactive non-killable NPCs, thus the player can not currently complete the Risk Life for Limb quest.
- Returning only four of the invoker jewels (ie. before you get the fifth one after killing Stone) to the control panel may prevent you from getting the small chest mentioned above. Bringing the last invoker jewel by itself to the control panel did not prompt the character to place the jewel on the control panel