Structure erected to strengthen a military position against attack. The defense of cities and trade centers, usually by high walls, has been important for centuries. The citadel was the fortress of the ancient world, appearing in cities of Egypt, Greece, and the Roman empire. By Classical Greek times, fortress architecture began incorporating ramparts (walled embankments) and towers. Roman fortresses of the 2nd century tended to be square or rectangular, and were usually of dressed (cut) stone. The medieval castle remained almost impregnable until gunpowder came into use.