THE 1539 TOWER - If you go down the ramp from Level III, you will be inside the oldest part of El Morro. When Charles V authorized the defense of the port of San Juan, this round turret style tower was built at the base of the promontory. It was only large enough for 4 small cannon. The original tower was swallowed up inside thicker walls when El Morro was expanded. The large shell fragment you see high on the ceiling dates from the bombardment of San Juan by the US Navy in 1898, when this part of El Morro was heavily damaged. The stairs descend to the Water Battery.