I have now been to both Uxmal and Chichen dozens of times, and I can honestly say that Uxmal does it for me. From the Magician's pyramid (clap your hands and hear the bird call), via the Nunnery Quadrangle with its incredible decorations, to the Palace of the governor and even the Dovecote, it is simply stunning. Although it spread over many square kilometres (37 I think) and had 25,000 inhabitants, a smaller area than at Chichen has been excavated and restored and the buildings are all closer together, making it less tiring to visit. And yes, there is a pyramid you can climb and enjoy the view from, so trainers or plimsolls would be a good idea. And unlike at Chichen, your enjoyment is not spoilt by endless vendours hawking their cheap souvenirs. There are only a few stands, and they are outside the site. A bottle of water, hat and mosquito spray are a good idea, and you can buy drinks and some souvenirs inside the building at the entrance where the bathrooms (impeccably clean) are also located. Of all the Maya ruins in the Yucatan (and I have visited most of them), Uxmal for me is the most impressive.
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