Xochicalco is a UNESCO world heritage site near the state capital Cuernavaca of Morelos in Mexico. It's around 40 minutes by car or 1.5 hours by bus (Lasser - MXN$21) from Cuernavaca Casino bus terminal, which itself is around 1.5 hours away from the Mexico City southern bus terminal by bus (MXN$165 - Pullman de Morelos).
Xochicalco is the only example in the world of an epiclassical fortified city, with 4 main parts (some of which are not accessible). there are 2 main sites in this large archaeological site, the great pyramid as well as the main plaza, and there's also a site museum.
It was the successor to the Teotihuacan culture, around the year 650-1000, and most likely collapsed after an internal revolt, as valuable items can be seen shattered and scattered around the area of the rich and powerful, while the residential areas of the lower classes had signs of a gradual abandonment instead.