Evange's post in [BUG][Od Nua][Lvl13] Mechanism for opening Wall is not clickable was marked as the answer
When you killed the Old Watcher you have 3 options:
1. Release his soul
2. Bind his soul to the keep
3. Absorb his soul
If you chose option 3 then you'll be able to access the secret room. Not sure what's in it though.