A engineer soldier is not a original soldier unit from the game, The Conquerors. They aren't seen used until a person has researched a wall.
A engineer is always used for making walls. Without a engineer a wall cannot be made, even if researched . They are mostly used in either the beginning or middle of a game due to having a defense, if there is no truce available. To make a wall, put a engineer between two wall columns that are within range of each other (or else it will not work) and it should automatically make a wall.
Due to engineers having no attack or healing ability, they are mostly only used for walls. Keep them near walls at all times if you want to repair a wall quickly..