I know about slip and the work Kenneth and guys are doing over there! Slip is amazing. Codedamn gives a lot more control over interactivity at an expense of a small learning curve. Try both and work with the one that fits you
We also have a free plan if you don't want to keep it under your own branding on your own domain. The $499/month (or any number we decide) plan is really for creators with high volume of sale (making $$$$/month) and don't want to pay 10% fee for every sale.
This is a relatively simple idea BUT do checkout "how" people have to attempt and submit challenges - on codedamn's in-browser IDE system - no downloads required for multi-file projects!