The Prague astronomical clock could be seen from the window of our hotel in Prague. It is, according to Wikipedia, the oldest working astronomical clock in the world. It attracts a crowd on the hour to watch The Walk of the Apostles as the clock chimes. The clock continues to chime through the night, though I cannot testify to the size of its audience at 4am.
The clock is mounted on the south wall of the Old Town Hall, the roof of which you can visit for a small fee. The town hall is on one corner of the Old Town Square where the visitor can find several sites and buildings of interest—including when you get tired of all the culture and history plenty of bars and restaurants to feed the outer man.