His Secret Service (Side Quest)
Pick this one up while freeing the prisoners in Chersonesos Roman Fort.
You can't climb, exactly, while carrying the quest-linked prisoner, but you can hop up the bale just to the left of the prisoner gate as you exit, and drop over the fence.
Get some distance from the camp, then speak to your rescuee- a secret agent. He'll give you an update on other spies in danger.
This will send you south, to one of the border forts (Bemulous Roman Tower, Livius Roman Tower, Hydrax Roman Camp, or Akabis Roman Tower. Possibly Thintis as well.). Wherever the local treasure is kept, seems to be the place the Centurion gravitates to. Track him down and kill him for more info.
This kill completes the quest.
It also looks like it was meant to unlock the Shadows of Apollo side quest, if that one didn't already populate during your previous trips through Balagrae.