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This will steal tiles from rival players and even city-states so long as there is no completed district or wonder currently on the tile. Of note: if you are playing as Poland, each fort you construct will also Culture Bomb surrounding tiles.
Forts are great defensive structures, especially for a defensively-playing civilization. They will spend 1 Iron and Coal, but they will be able to build as many railroads as you wish, both on existing roads (replacing the road) and on wild land. In Gathering Storm, as soon as you research Steam Power and unlock railroads, Military Engineers become able to build railroads without using build charges.
Military Engineer charges would be better used in creating bridges across Rivers, allowing your Builders to traverse your city using less MPs. Building roads between your cities with Military Engineers is expensive, it is better to use Traders for this purpose. And if it so happens that the Traders haven't happened to pass through this exact location, Military Engineers are your only option. Building roads through terrain which will slow down your army is now more important than ever. What's more, Military Engineers have both offensive and defensive uses: Their use isn't immediately apparent to any but the most skilled commanders, who know the value of a good road leading close to your target, or a well-placed Airstrip far from your cities, which your air force can use as a base for attacks. Unlike other support units, Military Engineers are active, not passive, participants in the game. Military Engineers are another type of Builder, focused on military and infrastructural improvements.
Military Engineers cannot be Captured by the enemy they will instead retreat to the nearest city, like Great People.
Can Repair Tile Improvements (requires no charge). Can Remove Tile Improvements (requires no charge). Can spend a charge to complete 20% of an engineering type of district ( Aqueduct, Bath, Canal, Dam) and Flood Barrier building. Can construct Railroads (requires no charge) and Mountain Tunnels. Can construct Roads, Forts, Airstrips, and Missile Silos. In the Gathering Storm expansion, the Military Engineer's Gold maintenance cost is decreased from 4 to 2. It can only be built in a city that has an Encampment with an Armory. The Military Engineer is a Medieval Era support unit in Civilization VI. Can construct Roads, Forts, Airstrip and Missile Silos improvements.