Towers were Used As Citadels in Time of War
Source: TTT
Judges 9:51
But there was a strong tower inside the city, and all the men, women, and leaders of the city fled there. They locked themselves in and went up to the roof of the tower.
Ezekiel 27:11
Men of Arvad and Helech manned your walls all around, and the men of Gammad were in your towers. They hung their shields around your walls; they perfected your beauty.