As previously mentioned, I dont actually remember which fortress its found. But there only 1 general which make you chase him into the arena in flying fortress, where 1 invoker jewels can be found after defeating him. After getting out from the arena, you will be back inside the flying fortress and I sure there are still unexplored area, if you just follow the general straight to the arena. The rest of the invoker jewels are in that unexplored part of the fortress, which I dont think its hard to be locate. You can actually forget bout it if lazy, as the reward is not something important also if I remember correctly.
