Slitherine

I thought i captured everything and even got the bonus for having all Army of Africa troops in Seville and it's airport.
But the scenario ended in my Defeat.
I suppose it's the city of Cadiz, there's a flag there, but i was unable to move a unit onto it.
I think it's a sort of Supply hex and i was able to bombard it with tac bombers but never captured it.
Is there a special unit i need to use?

There's a 2-strength supply depot at Cadiz that needs to be destroyed before you can occupy it, but any ground unit can do that.