https://www.youtube.com/watch?v=0xBJwrm9C8w
https://www.gdcvault.com/play/1023139/Level-Design...
https://www.rockpapershotgun.com/2015/01/22/how-to...
https://www.gamasutra.com/blogs/AsherEinhorn/20150...
There is another book "the Fundamentals of puzzle and casual game design", but I have not seen in person.
Without beta testers anywhere. He didn't do levels, but one friend drew a rather big mod to Braid — beta testers had served his father and me.
Here are found in Braid:
gamedesignreviews.com/reviews/braid-understanding-...