Gen Con has been running about 60,000 people a year for the last few years; unofficial word is that it spike to 70,000 this year. You can see historic attendance at https://en.wikipedia.org/wiki/Gen_Con#Timeline The card buildings are "Cardhalla". I believe Cards Against Humanity sponsored this year, although I'm not actually sure what that entails. Anyone can build for free. On Saturday evening there is an auction to throw the first coin at the construction; we were wandering by and I seem to recall bidding was at $400. Once the first coin is thrown, everyone is welcome to throw coins. After the destruction, the coins and winning bid are collected and donated to charity. The very first Cardhalla was entirely a fluke; people started building with cards that companies were giving away as promotions, and it grew to surprising size. Someone had the impromptu insight to turn it into a charity, and auctioned off the right the stomp through it like Godzilla, but the person who won said everyone was welcome to throw coins at it instead. By the next year it was a formally organized event.