This is a great experience but if you opt to ride the walls (13.7 kms) be aware that there is significant earthquake damage in parts and stones missing all over the place so it's a really bumpy ride. You need to stop and look at the views as you dare not take your eyes off the surface ahead. Bikes are sturdy (necessarily!) and in good condition and quite cheap for such a unique experience. We took an hour and 20 minutes to cover the four walls and that was travelling fairly quickly with a few photo stops along the way. Would recommend others allow more time. Some nice bars below when you get off the bike! Great experience.