Rommel believed that the Normandy coast could be a possible landing point for the invasion, so he ordered the construction of extensive defensive works along that shore. In addition to concrete gun-emplacements at strategic points along the coast, he ordered wooden stakes, metal tripods, mines, and large anti-tank obstacles to be placed on the beach to delay the approach of landing craft and to impede the movement of tanks. Expecting the Allies to land at high tide so that the infantry would spend less time exposed on the beach, he ordered many of these obstacles to be placed at the high-tide mark. Tangles of barbed wire, booby traps, and the removal of ground cover made the approach hazardous for infantry. On Rommel's order, the number of mines along the coast was tripled. Given the Allied air supremacy (4,029 Allied aircraft assigned to operations in Normandy plus 5,514 aircraft assigned to bombing and defence, versus 570 Luftwaffe planes stationed in France and the Low Countries), booby-trapped stakes known as "Rommelspargel" (Rommel's asparagus) were set up in meadows and fields to deter airborne landings.