Does anyone have an opinion regarding tickets for this site regarding, whether to purchase just the basic tickets;
or would it be worthwhile to do a guided tour to experience to whole Colosseum - the underground arena as well?
Thank you
EA
Recent similar question
https://community.ricksteves.com/travel-forum/italy/best-way-yo-see-the-coluseum
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The underground tickets are the hardest tickets to secure. In fact, any Coliseum ticket can be difficult, with the current system in place.
Good luck!
You can easily get the generic tickets. If you want an experience you'll have to get up super early on the day they go on sale. I think they open at 8:30am Rome time, so just figure out what time you need to get up to buy them. Even if you miss out, sometimes the night experiences don't sell out as fast. One time we got regular tickets for one day and night tickets for the underground a few days later. It was worth it and much cheaper than booking through a 3rd party tour company.
I don't encourage setting your heart on the underground area. It is very unlikely to work out through a tour group. That said the regular colosseum/forum tours are great and I would go with a guide.
When we visited in 2014, the underground tour wasn't an option, but I can tell you that we took a guided tour and thought that the Forum and Palatine Hill were far more interesting than the Colosseum.
We just did an incredible guided tour, hour long, with a great guide (Agostino) who met us at 7:30am, we took the elevator to one of the top decks (didn't even know they had one!) and saw the whole thing mostly from the top and some of the side pieces. It was entirely empty save just a few people. It does not go underground. But really I find it hard to believe it could possibly be better than what we saw. They limit it the number of people who can come in before the whole thing opens. It was a bit of a splurge - but really an incredible experience. This is his tour group: https://www.kirbatours.com/