The game was developed by Mind Candy. I had met one of the directors, Michael Acton Smith (who later went on to design Moshi Monsters) and thought we was switched on, so I bought a pack of Perplex City cards to see what they were about.
I was already familiar with ARGs, because Jess Mulligan had described the concept to me. She'd previously pitched the idea to Electronic Arts, who proclaimed it to be a bad idea and then went on to develop Majestic. If Majestic had been a commercial success, Jess might have sued, but it wasn't. EA didn't really understand what they had.
Anyway, the cards all turned out to be puzzles. As is invariably the case with puzzles that offer big prizes for solving them, there are always going to be a few that have multiple solutions depending on how you read them and some that are so obscure that it's pretty well guesswork as to what they mean. One, which asked players to solve the Riemann hypothesis, would win an Abel Prize for anyone who managed to do it.
Majestic wasn't a success because you had to pay $9.95 for four months to play, so it didn't rack up enough players. Perplex City was more successful because it sold physical cards, but it still didn't bring in enough money to make ARG-creation a worthwhile business.
You can still buy Perplex City cards on eBay. They sell for about £2 each. Here's a scan of the ones that were in the pack I bought:
I used to keep them in their foil packet pinned to the noticeboard in my office at the university. Unfortunately, the actions of the sun through long periods of lockdown rather faded it somewhat. Still, the contents remain fine.
If I weren't so lazy, I'd sell them.