While colleges were the primary location that theater started in Colorado, the Auditorium was a magnificent structure first built in 1908. William Jennings Bryan was nominated to run as the Democratic candidate in one of the first events there. The building could initially host any event from opera to rodeo. The seating capacity was flexible, from 3,000 to 12,000.

There have been several renovations to the building, adapting it from the original flexible space to separate theater and arena spaces, then eventually to an opera house. The building was opened as the Ellie Caulkins Opera House in 2005, which currently seats 2,200 patrons and hosts many opera and musical performances today. Here is a good Wikipedia article article on the full history of the building.