This Swabian wall replaces the previous Norman wall and it’s located in a more advance position. The towers are apparently equal, but they had different uses as ascents to the stands, housing ammunition, but all were called to the guard from a walkway perimeter placed at the highest level. 

Terranova C.P. La città murata Dalla visita al "memorabile viaggio.", in Milazzo Nostra n. 28-29 - Marzo - Luglio 2011, pp. 24-25