Easy and Medium are for beginners and kids, and Hard and Expert are for players who know the basics. From Master up, the computer uses the open-source Marcher Engine, one of the strongest freely available checkers programs, and each level unlocks when you beat the one below.
Can I take back a move?
Yes. Undo takes back your last move and the computer's reply, and redo puts it back. Using undo or the hint still counts – you can unlock the next level with them.
Do I need an account or a download?
No. Everything runs in your browser on phone, tablet or PC, and your current game, settings and unlocked levels are saved on your device automatically.