 Safety barriers are often arranged in chains. Ideally, each barrier is independent of its predecessors so that if the first fails, then the second is still intact, etc. For example, in an ancient fortress, if the enemy managed to pass the first wall, then additional layers would protect the defending forces. Some engineering safety barriers follow the same principle of concentric physical barriers. Others are arranged serially in a temporal or functional rather than a spatial sense. One of the lessons that engineers learned from the Titanic disaster is that improved construction of early barriers is not of much help if it leads to neglect of the later barriers (in that case lifeboats).