There should be a couple Black Ring soldiers when you enter. After talking to Williams and getting the key for the mine (any you can pick up are for chests in the tower), he and the other miners decide to rush downstairs as 4 Black Ring archers come up, with 3 more soldiers on the bottom floor.
Can you use the dragon stone to leave? (probably not) In DKS the tower should be locked until after you have the battle tower, though in the original release of D2:ED you could enter early. Also, in DKS the doors get locked until you defeat all the Black Ring members, while in D2:ED if you got past the archers you could leave by a side door and avoid the soldiers on the first floor.
Do you have a save before entering the tower, or before talking to Williams?
If there was a glitch either with Williams running downstairs or some of the Black Ring opponents, reloading a previous save may get it working.
If not, try emailing support@larian.com, with a description of the problem and maybe a link to this topic. See the DKS
support page.
In XP the saves are in the folder
C:\Documents and Settings\ %username% \Local Settings\Application Data\Divinity 2\Savegames
or in Vista / Win 7
C:\Users\ %username% \AppData\Local\Divinity 2\Savegames
You may need to set Windows Explorer to show hidden files and folders (
XP, Vista,
Win 7).