Xochicalco was the third ruin I visited while in Mexico. As part of my quest, I visited several more. However, I still have a fond memory of the sight and feel it is deservedly listed as WHS.
Key reason for me that make the sight special is it's location. Built on top of a hill it feels like a fortress meant to protect its inhabitants. I was reminded of the Acropolis or a medieval castle. This is very different from most other ruins I saw in Mexico, apart from Monte Alban.
I also enjoyed the gardens greatly. Not sure why, but it seems they had a good gardener and the sight feels a bit like a park.
Logistics
Getting to Xochicalco is fairly straightforward. Good busses run hourly from Mexico City (South) to Cuernavaca. And similarly from Cuernavaca to the sight itself dropping you off at the entry to the museum.
Getting away from Xochicalco cost me 2.5h. Everyone told me the good bus that took me there would show every hour. It didn't show for two hours. My guess is that the drivers, don't really like taking the detour. So they skip on the stop and go directly to Cuernavaca. Instead, take the 3rd class bus to Cuernavaca from up the hill. It will not drop you at the right bus station in Cuernavaca and you have to make your way through the city to get to the Mexico City bus.