Adam Clarke on 1 Samuel 26:5

David arose - As David and his men knew the country, they had many advantages over Saul and his men; and no doubt could often watch them without being discovered.

Saul lay in the trench - The word במעגל bammaegal, which we translate in the trench, and in the margin in the midst of his carriages, is rendered by some in a ring of carriages, and by others in the circle, i.e., which was formed by his troops. Luther himself translates it wagenburg, a fortress formed of wagons or carriages.

As עגל agal signifies any thing round, it may here refer to a round pavilion or tent made for Saul, or else to the form of his camp. The Arabs, to the present day, always form a circle in their encampments, and put their principal officers in the center.