Old town is sorta ground zero in Heidelberg. We got off the Bahn at Alstadt stop--very unbusy as most visitors looked like they were 'locals' or on one of the dozens of tour busses that eventually arrived [I'd recommend NOT doing the latter but opt for a day trip on own--easy, cheaper, and time unrestrited]-- rather than the hbf and found the 1/3 mile a great way to stretch after the 1 hr train ride from Frankfurt hbf [think about 1.5-2.0 miles to hbf). The center of old town is nice and quiet before noon--before restaurants put tables, chairs, and umbrellas out and before the tour groups begin coming down from the castle; after it's people fest. Side streets are pedestrian friendly but other than the Church of Holy Spirit [look for white outline on ground near Kornmarket that demarcates where hospital used to be--kinda cool], there's not a lot of must-sees. Good for people-watching, cruising side streets, used as a meeting point, and as a central location between all other old town haunts.
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