The Military Engineer is a Medieval Era support unit in Civilization VI. It can only be built in a city that has an Encampment with an Armory.
In the Gathering Storm expansion, the Military Engineer's 
Gold maintenance cost is decreased from 4 to 2.
- Attributes:
- Gets 2
charges ( 4 if playing as England).
- Gets 2
- Abilities:
- Can construct Roads, Forts, Airstrips, and Missile Silos.
- Can construct Railroads and Mountain Tunnels.
- Can spend a charge to complete 20% of an engineering type of district (Aqueduct, Bath, Canal, Dam) and Flood Barrier building.
- Can Remove Tile Improvements (requires no charge)
- Can Repair Tile Improvements (requires no charge)
Military Engineers cannot be Captured by the enemy; they will instead retreat to the nearest city like Great People.
Strategy
Military Engineers are another type of Builder, focused on military and infrastructural improvements. Unlike other support units, Military Engineers are active, not passive, participants in the game. 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. What's more, Military Engineers have both offensive and defensive uses:
- Building roads through terrain which will slow down your army is now more important than ever. 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 between your cities with Military Engineers is expensive, but can allow you to use your Traders for trading with outside civilizations.
- In Gathering Storm, as soon as you research Steam Power and unlock railroads, Military Engineers become able to build roads for free...or rather, without using build charges. They will spend 1 Iron and Coal, but they will be able to build as many railroads as you wish, both on existing roads and on wild land.
- Forts are great defensive structures, especially for a defensively-playing civilization. Of note: if you are playing as Poland, each fort you construct will also Culture Bomb surrounding tiles. 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.
- Airstrips become very important when conducting offensive operations away from your main territory, especially on other continents. Conquered cities don't offer all the slots for air units that they did in Civilization V, and a quick Airstrip solves the problem of where to house bombers that are a long way from home.
Each Military Engineer gets only two uses before vanishing. Prioritize what you need them to do. If playing as England (Civ6) not only that your Military Engineers get four charges, you also get a 100% 
Production towards them! This allows you to gain up to 80% Production for Aqueducts, Canals, Dams, and Flood Barriers!
Civilopedia entry
Military engineers – as opposed to “combat” engineers who are crazy enough to rush into battle carrying tools and explosives rather than weapons – are tasked with building fortifications and other structures and creating and maintaining lines of supply and communication. The most victorious armies of the past – Roman, Chinese, Ottoman, British, etc. – all had superior military engineering units, to which much of their success was owed, both for protecting their own troops and knocking down the enemy’s defenses. Just ask the Gauls at Alesia (52 BC) or the Zealots at Jerusalem (70 AD). Following the fall of Rome in the west, military engineering – like just about everything else – entered a “dark age,” not to be revived until the mid-Middle Ages, when once again sieges were prevalent. By the end of the age, military engineers were again an integral part of any victorious army. Throughout the Napoleonic, Crimean, various civil and world wars, military engineers were divided into ever-more specialties. These days, a military professional needs a degree in civil, electrical, mechanical, architectural or even environmental engineering to be given the chance to build things...or tear them down.