Hanover was a city, and a city was, above all else, an organism for repelling armed assaults. The Leine, which flanked Hanover on the south and east, had always had some part to play in keeping the place from being sacked and burned. This explained why the Schloß rose straight from the river’s bank. But the precise nature of the Leine’s military duties had changed from century to century as artillery had gotten better, and gunners had learnt math.