Here's a tough one, I've looked for a very long time but it may not exist anymore. It could only be played online but was not a multiplayer game to my memory. It was either a 50 or 100 level tower in which you goal was to get to the top. You could choose one of three characters - fighter, acrobat (I think), or one other. It had a 3D perspective with a creepy, harlequin feel. After so many levels, you could save your progress and I believe there was a market every 10 levels. Also, some levels were all about trying to solve a puzzle to move on. It was very difficult, especially the bosses, but very addictive. I remember it have a weird name but just can't remember it.