La Specola is a tall fortified tower at the southwestern edge of the old town that has had a remarkable career: a medieval defensive tower of the Carraresi rulers, then a notorious prison ("the torture tower"), and from 1767 the astronomical observatory of the University of Padua. The conversion was the work of the abbot-astronomer Giuseppe Toaldo.
It is now an astronomy museum, with a fine collection of historical instruments — telescopes, quadrants, an enormous meridian circle — and access to the upper terraces with a sweeping view across Padua's terracotta roofs to the Euganean Hills. Visits are by guided tour at fixed times; the climb to the top is steep but worth it.