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The Fort is a defensive tile improvement in Civilization VI. It requires Siege Tactics and can be built by a Military Engineer.
The Roman Fort is identical in function to a normal Fort, but is available much earlier in the game (since it can be built by a Roman Legion).
- Effects:
- Occupying unit receives +4
Defense Strength and automatically gains 2 turns of fortification.
- Occupying unit receives +4
The Fort can be constructed on any of the following terrain and its (Hills) variant:
Certain terrain features, such as Woods or Rainforest, will have to be removed before the Fort can be built. Also note that a Fort may not be constructed on top of Resources.
Strategy
Attacking and expanding may be what wins games (especially for players who are trying for a Domination Victory), but defending well can deter aggressive players and make them think twice about launching an attack on you. This is when Forts come in handy. Have some Military Engineers build them at choke points in your territory or in other strategic locations along your borders, then station some units there to fight off any invaders. Place Forts on hills for best results.
A Fort grants the unit on it a total of +10 
Defense Strength, regardless of who builds it and whose unit is on it. A poorly placed Fort (e.g. one in the middle of open flat land) can easily be swarmed by enemies and used in their favor.
Poland can build Forts to annex the surrounding tiles, thus giving them a way to claim new territory and steal resources from opponents who settle nearby while also converting their cities to Jadwiga's religion. The Poles should always keep a few Military Engineers around to use for this purpose.
Civilopedia entry
Since the first ego-maniacal chieftain threw up a wall of dirt and proclaimed everything behind it his, soldiers have been building fortifications. Whether to pacify new territory, protect the borders, or insure that the ruled didn’t get uppity, the military-minded have dug ditches and moats, planted stakes and spikes, and put up walls of all sorts. Some made such military engineering an art form, such as the Roman legions, which could march through the day and have a fortified camp in place by nightfall. In time, the appearance of gunpowder led to innovations in fortifications; those wooden and stone walls were simply no longer adequate. So, rather than build up, fortifications went down – trenches and bunkers, pillboxes and minefields, barbed wire and “defense in depth.” But the intent remains the same, to protect the troops and keep the civilization safe from both without and within.