This section is important as showing that David, the younger son of a not very opulent family 1Sa 16:11, 1 Samuel 16:20, had now become a large landed proprietor, as well as a capitalist, possessed of much moveable wealth. We may perhaps see the sources of both these kinds of property, in the successful wars which he had waged 1 Samuel 27:8-9; 1 Samuel 30:20; 2 Samuel 8:4, 2Sa 8:7-8, 2 Samuel 8:12; in the revenue derived from subject kings 1 Samuel 8:2, 1 Samuel 8:14; 1 Samuel 10:19; and in the purchase and occupation of lands in different places. Further, he enjoyed, of course, the usual rights of a Jewish king over the landed property of his subjects, and was thus entitled to receive a tithe of the produce in tithes (1 Samuel 8:15, 1 Samuel 8:17) and in “benevolences.” Compare 1 Samuel 10:27; 1 Samuel 16:20, etc.
The castles - Probably the watchtowers in the border districts, exposed to raids from the plundering tribes of the desert 2 Chronicles 26:10; 2 Chronicles 27:4.
In the low plains - Rather, “in the Shephelah,” the proper name of the low tract between the hill country of Judaea and the Mediterranean.