The canal came together with others, and at some point they were on the river Amstel, which took them into the place, just short of its collision with the river Ij, where it had long ago been dammed up by the beaver-like Dutchmen. Then (as Jack the veteran reader of fortifications could see), as stealable objects, lootable churches, and rapable women had accumulated around this Amstel-Dam, those who had the most to lose had created Lines of Circumvallation. To the north, the broad Ij – more an arm of the sea than a proper river – served as a kind of moat. But on the landward side they'd thrown up walls, surrounding Amstel-Dam in a U, the prongs of the U touching the Ij to either side of where the Amstel joined it, and the bend at the bottom of the U crossing the Amstel upstream of the Dam. The dirt for the walls had to come from somewhere. Lacking hills, they'd taken it from excavations, which conveniently filled with ground-water to become moats. But to the avid Dutch there was no moat that could not be put to work as a canal. As the land inside each U had filled up with buildings, newly arrived strivers had put up buildings outside the walls, making it necessary to create new, larger Us encompassing the old. The city was like a tree, as long as it lived surrounding its core with new growth. Outer layers were big, the canals widely spaced, but in the middle of town they were only a stone's throw apart, so that Jack and Eliza were always crossing over cleverly counter-weighted drawbridges. As they did so they stared up and down the canals, carpeted with low boats that could skim underneath the bridges, and (on the Amstel, and some larger canals) creaking sloops with collapsible masts. Even the small boats could carry enormous loads below the water-line. The canals and the boats explained, then, why it was possible to move about in Amsterdam at all: the torrent of cargo that clogged roads in the countryside was here transferred to boats, and the streets, for the most part, opened to people.