Amazing
This was one of the best games I've ever played. It looks like you're hiding behind the style of nostalgic 90s games to appeal to players that grew up around that time, but you really don't need to. Even if this game had modern-looking graphics, the simple gameplay would still easily annihilate the competition.
Graphics were pretty good. The pixel art is way better than I could do, but the lighting on the ladders looked kind of weird. You might wanna think of polishing that up. Also, I don't know if the little white guy you control was a tribute to an existing character, but if not, he needed more detail.
The music got kind of repetitive, but I actually liked the sound effects (even if they got repetitive too). I'd personally suggest adding more songs tacked onto the current one, maybe with a random playlist or something to spice it up. Kraftwerk and Nullsleep are huge recommendations here, although you might also consider Nine Inch Nails if you're appealing to the demographic into the whole sprites, big headphones, 8-bit, etc.
The gameplay was pretty interesting. Tower of Greed is a game of risk and reward, security and insecurity. While you had a sense that everything could go down the drain instantly, you also got immense satisfaction from locking in a medal. The player moved great; I loved how fast he could move. Also, the gems were placed pretty well. It was usually just out of your way to lure you on a long chase.
There were a couple problems I had with it, though. For one, a few diamonds or items were extremely hard to pick up without getting killed, when diamonds and items with the exact same value could often be obtained by easier means. To remedy this, it would have been nice if you had an extra kind of gem (like a red ruby) to rectify the strange learning curve. It would have made it easier to give better rewards for harder maneuvers.
Also, the jumping enemy and stair-climbing enemy were pretty unbalanced. Sometimes they'd jump right off the screen, but sometimes, either at a low level (like 4) or a high level (like 40), one of them would literally stay on top of me to the point where i couldn't shake him and I kept getting hurt until I died. This is a very, very serious problem that can slash a player unfairly.
The shield powerup was a bit useless, meaning that 1/4 of the time you picked up an item it was fairly useless. I also couldn't see the difference between survival and arcade.
Lastly, it would have been nice if you had an adrenaline mode where the stage moved a bit faster. I found myself craving action sometimes and being denied it, but would have loved something where you could barely keep up with the pace of it all.