Jumpman (no relationship to Randy Glover's 1980s title) is in many ways a classic, old-school platformer, but with some interesting design twists.
Its graphics are attractive but pixel-blocky. Gameplay is mainly move-and-jump, with the usual "get to the exit" approach of level-based platformers, but movement feel takes some getting used to. In essence, holding down left or right accelerates you, so the longer you press the key down the faster you move, and speed slowly diminishes, as it might in the presence of air resistance. The result is that you have to unlearn the instincts taught by classic platformers, in which movement is at constant speed, and it takes some time to master control over movement.
The problem is complicated by the fact that the processing loop of the game is visible -- if you slide slowly left or right for some distance, you'll see your character moving a little faster then a little slower, as other moving objects on the screen update. This makes it a little harder to judge the controls, and is -- well, sloppy; it would be possible to code the game to eliminate this.
One interesting aspect is the upcoming levels are visible behind you as background to the current one, which is visually a nice touch, making the screen far less stark than it otherwise would be.
In addition to movement and jumping, you can rotate the entire screen clockwise or counterclockwise (a and d keys in the default control configuration), with, as is typical in games that permit this, 'gravity' always being toward screen bottom. You don't need this for the first few levels, but later ones require this to solve the problem. And of course, new entities are added over time -- e.g., large balls that can be rolled into enemies to squash them (but will also squash you), tumbling blocks that can open or block paths with correct screen rotation, and so on. The result is inventive level design and some interesting challenges.