Query: What was the name of the tower that was the western boundary?
Context: The city grew out towards the west of the boundaries of the peninsula formed by the River Aare. The Zytglogge tower marked the western boundary of the city from 1191 until 1256, when the Käfigturm took over this role until 1345. It was, in turn, succeeded by the Christoffelturm (formerly located close to the site of the modern-day railway station) until 1622.