Quran by dr Abdul Aziz

They convinced them to attack the prophet and his followers (before the battle) when the prophet knew they were coming, he gathered his sahabah and asked for advice, and one of them was "build a ditch/trench" so that they could protect Medinah.

Medinah was protected by mountains on the left and right and dense date plantations from the south, treason from the inside by the Hypocrites, this battle revealed the true believers from the non-believers.

The sahabah guarded the trench, if the attacked tried to jump the gap, first they would fall into the trench, and two they would get shot by arrows by the sahabah from the other side of the trench.