Zytglogge was built around 1218–20, the Zytglogge served as the gate tower of Bern's western fortifications. These were erected after the city's first westward expansion following its de facto independence from the Empire. At that time, the Zytglogge was a squat building of only 16 metres (52 ft) in height. When the rapid growth of the city and the further expansion of the fortifications (up to the Käfigturm) relegated the tower to second-line status at around 1270–75, it was heightened by 7 metres (23 ft) to overlook the surrounding houses.Only after the city's western defences were extended again in 1344–46 up to the now-destroyed Christoffelturm, the Zytglogge was converted to a women's prison, notably housing Pfaffendirnen – "priests' whores", women convicted of sexual relations with clerics. At this time, the Zytglogge also received its first slanted roof.