Charles H. Spurgeon on Psalms 48:13

Mark ye well her bulwarks. Margin as in the Hebrew, "Set your heart to her bulwarks." That is, pay close attention to them; make the investigation with care, not as one does whose heart is not in the thing, and who does it negligently. The word rendered bulwarks, (lyx), khail —means properly, a host or army, and then a fortification or entrenchment, especially the ditch or trench, with the low wall or breastwork which surrounds it. 2Sa 20:15 Isa 26:1. (Gesenius, Lex.) —Albert Barnes.