Are these sentences correctly worded?

FYI- The three legions are the Council's forces/army, the northerners are Szass Tam's army, and Szass Tam is the undead wizard. The legions of Eltabbar, Pyarados, and Tyraturos used powerful magic to conceal their advance, and when they assembled at the Keep of Sorrows, they believed that they had successfully surprised Szass Tam’s host and had trapped the northerners between themselves and the fortress. The Council of Zulkirs’ forces immediately attacked the undead wizard’s army and drove them toward the castle, where the defenders assailed them from the walls. However, as the Council’s troops continually pushed harder against the northern legions, Szass Tam discerned that they were too entangled with his followers to withdraw and launched his trap.