The Construction Soldier is a original unit from the game, The Conquerors. They are used to construct buildings in remote locations, often away from energy crystals. They automatically spawn at the start of a game in a water map, along with 3 Light Soldiers and a Scout, however, the player will start with none on land Conquest maps, one on land Territory Conquest maps, and two on ocean Territory Conquest maps.
Construction soldiers can be made to deploy (hotkey N) a construction yard. This process cannot be reversed. A construction yard is a simple structure that does nothing except allow other buildings to be built nearby, allowing the player to get buildings in places that are away from any energy crystal or from any other structures. Due to locations without energy crystals not being of much strategic value in general, they are mostly used to create shipyards.
In the beginning of games, most people use a Construction Soldier to make a Naval Shipyard if there is water, so they can make an Oil Rig, Power Plant, or Nuclear Plant on another island. They are rarely used during the middle or at the end of a game, as people will already have Airports and Space Links, although they can be used to establish a forward base.
As the Construction Soldier can let you build most things in most places, They are ideal for blocking off choke points, such as: bridges, small areas the enemy could use to reach your base or a tactical area. They are also good for setting up an outpost in a tactical area or for "Proxy Building" (building very close to the enemies base.)