It seems we were entering on the side that used to be the stage. Inside, it was totally different than I'd pictured it all these years. I was thinking ruins, but in fact the arena had been built into a playground where adults and kids were all able to use it for free, and several different types of games were going on at one time. There are a lot of nice photos of this place on the web, and I'll add mine as well. We sat on a bench just inside the entrance, and the photos show the arena from left to right.
It's a good thing to know where the public bathrooms are when you're travelling. There's one in about the center of the photo up on the grassy area.