The Construction Crane unlocks an extra construction slot at the castle in which it is built. The extra slots (1 or 2) are functioning as a "queue" for construction, which is always done one at a time from the "primary" slot only. When there is an extra slot and the "primary" slot is free (finished), the building at the 1st extra slot will move to the primary and the building at the 2nd extra slot will move to the 1st slot. This will make easier for massive building (for novice players) or to repair buildings on fire at wartime.