The green arrow marks the district that belongs to you.
When you enter the district view, the two individuals in the front are the ones who are defending this area. In an Action match, you and another gangster will share a tower to earn season points - and the one with more damage will earn the extra points. Other gangsters doing more damage than the two individuals who own the tower will not impact where the rewards go to.
Critical attacks in the district can only be blocked by the owners of the district who have protective vests.