Civilizations have always constructed fortifications to defend against barbarian raids and war. Simple constructions could withstand the primitive assault weaponry of early warfare. But as assault tactics and weaponry advanced, engineers had to build stronger fortifications like castles. Castle improvements were made faster than weapon advancements, which made castles the predominant fortified structure for many centuries. As tactics and weaponry continued to improve, the superiority of the castle faded. Aircraft rendered castles obsolete in the 20th century. But the basics of fortification continued through the nuclear age into the 22nd century with the development of bomb shelters, underground installations, and undersea bases.