FreeCol:Colopedia.concepts.fortification.description/en

Units can fortify themselves on a tile (by building temporary trenches and barriers) to increase their ability to defend. To fully fortify takes one turn. Fortification also prevents units from being ambushed, and fortified artillery are no longer considered to be "in the open" although attacking will damage the fortifications. Fortification only provides a defensive combat bonus in the absence of other favourable defensive features, thus no bonus is available when fortifying on mountains or in a colony with a fort or fortress.