Harbor is a medium sized map that often feels much smaller due to its tight design. The north and west are mazes of shipping crates and the map's center is crammed with narrow alleyways and buildings with small interiors. The wide street between Warehouse and Market in the south provides the most open space, but even so there are several nooks and alleys from which an attack can come at any time.
Expect a lot of action between middle map's Apartments and the Fisherman's Catch building. They're close together with a first and second story where firefights constantly erupt between them, especially over the middle alley by Forklift where doors and windows directly face each other.
Success in Harbor means striking the right balance between speed and caution. If you're running around corners in a full sprint, you'll lose a lot of fights. On the other hand, sticking around one place for too long invites trouble.
Keep moving but be tactical. Stick close to crates and nooks in the walls to cover your sides and back when moving forward. Raise your weapon as you turn corners or center your crosshairs so you're ready to hip-fire at a moment's notice. You've got to be ready for a fight at any time.
It's not all close-quarters combat, though. Snipers and other long-range players can do plenty of damage covering the streets and alleyways. There are some long sightlines to find around the perimeter of the map looking in. If you're playing the long game, consider bringing a trusted secondary with you to fight off enemies at close range.
There are plenty of climbable objects scattered throughout Harbor that when used tactically can give you a great view over the maze-like ground floor. And when you're under attack, use the plentiful cover to move out of view and then turn the attack around.