Robber

Bob the Robber 3 is the cleanest introduction, and the season entries build on it without changing what you've learned. Any of them works as a starting point.

No. Each is self-contained with its own setting and levels, so nothing carries over that you would miss. The numbering reflects release order rather than a story you need to follow.

Yes, in a puzzle form. You're avoiding guards whose patrols follow set patterns, so it's about observing and timing rather than reacting - being spotted usually restarts the section.

Each lock is a small minigame rather than an automatic action, so it takes time you have to buy by getting the guard timing right first. Rushing one is usually how a level goes wrong.

Yes, all of them run free in your browser with nothing to install and no account needed before you start, including all three entries in the series.