This very small archaeological site, 1km northeast of the center, actually has two pyramids, one inside the other. You can climb on the outer base and see the older pyramid within, with a double staircase leading up to the remains of a pair of temples. Tlahuicas built the older pyramid over 800 years ago; the outside one was being constructed by the Aztecs when Cortés arrived, and was never completed. The name Teopanzolco means ‘Place of the Old Temple.'
There are a few explanatory signs in English. The site was damaged by the earthquake of September 19, 2017. At the time of writing it was closed for restoration work. Check before your visit to ensure that is has completely opened again.