The engineer unit is an original soldier unit from the game, The Conquerors. They cannot be produced until you've researched walls.
The engineer is used for making walls between wall columns. Without an 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 the engineer near two wall columns that are within range of each other, and the engineer will begin creating the 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.