The game has a 2D RPG skin with cute characters and runs directly in the browser. At the beginning it is a bit hard to get used to its UX and for those who are anxious, sorry to tell, but you will have to read the instructions. But when you start to get how it works it gets more fun and without noticing you are learning how to code and hacking the code they give you in order to pass the levels faster.
The better part of Codecombat I just found out yesterday: the team who built it has open-sourced everything! I have even talked with the team and they are super accessible, they have provided a few aways of doing that.
There are a bunch of ways to get involved with the game and helping spread code education out there. So, if you are looking for a free and open-source software to contribute, you should consider this one.