Prinny's "catch" is that you have 1000 Prinnies (or lives) to make it through the entire game. Though it may sound like the trappings of an easy game, it isn't. The game practically boasts about its difficulty level from the minute you start it up.
Prinny offers two difficulty levels, Standard and Hell's Finest. Standard is for "casual" players and allows Prinnies to take three hits before dying (which works out to 3000 lives if you want to do the math). On Hell's Finest, Prinnies are, well... they're Prinnies - one hit and it's all over.
Prinny features ten levels, six that are available from the start and another four that unlock later. There's also a day/ night cycle that plays into the game's difficulty level. During the day, levels are "easier" while at night, levels are really freakin' hard. Obviously, enemies are harder to defeat later in the day, but the level's geography will also change. Platforms that were around earlier in the day will disappear, making the platforming aspects incredibly tough. It never gets to the point that it's cheap, but it won't take long for you to start wishing you had a few more lives.
