On 13 June, the French began to prepare large amounts of wood and digging materials on the west-side of the city. The north of the fortress was protected by a deep and wide moat, directly connected to the river Maas, while the south was covered by the Jeker rivulet, which would flood trenches. The obvious attack route therefore was from the west, over the high and rocky ridge leading to the ford that had in Roman times been the origin of the settlement. Frederick Henry in 1632 had attacked this side also. The drawback of the location was that deeper trenches of the type de Vauban preferred had to be dug through more or less solid rock; this was still feasible, however, because the layers consisted of relatively soft marlstone. In this sector two main gates were present, the Brussels Gate in the north and the Tongeren Gate ("Tongersepoort", "Porte Tongres") in the south.