A siege is a military term meaning to surround a fortress and try to cut off its connection to supplies and reinforcements. Sieges are often done during the earlier days of war the fortresses of the generals were extremely huge and hard to bring down. Before the invention of missiles and bombs, warriors had to rely on arrows, swords, catapults, and other kinds of these weapons. So in order to capture an enemy fortress, they had to surround it first and then infiltrate it after because they cannot destroy it easily. This is what is called a siege.