I had heard negative comments about guides and we had the luxury of taking two days to visit the area. The second day we got a guide outside the gate, talked with her a little before we went in, just to make sure this was not a canned tour. It wasn't! The official guides have attended classes in the university and beyond that are knowledgeable and passionate.
For us it was nice contrast - wandering through the site based on our prior readings and then being able to ask someone questions we had.
Use the facilities before you go in (bring 1 sol). This is an archaeological site, not a tourist attraction. You can come back out to use the facilities or eat your lunch - expensive if you don't bring your own. Make sure to bring water - it can be hot!
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