Complex sentence: Marching overland with them, he managed to surprise the besiegers and scatter them in a night attack, but on the next day they regrouped and drove Junayd and his men back into the fortress.
Split into simpler sentences: Marching overland with them, Junayd surprise the besiegers and scatter them in a night attack. On the next day though, they regrouped and drove Junayd and his men back into the fortress.