After visiting Heidelberg Castle, I was getting ready to head back down to town via the cable car when I found a set of stairs, seemingly going down forever.
I had a train to catch back to Frankfurt that night and didn't want to get lost, but decided to give the mystery stairs a go. Several flights later (and I mean several!) I found myself in the heart of the town beneath the castle. It was beautiful and felt like such a hidden secret that I just walked around smiling, swinging into shops, and sitting in the square just soaking in the vibe of the city.
If you're not physically prepared to climb all those steps up to the castle, take the cable car. But, I highly recommend taking the stairs back down to earth after you explore the ruins! You see a completely different end of town you would normally miss if you take the cable car.