I am having a hard time with some of the levels that have a hole in the floor... like Backup (18) and Errata (20). Is there a trick to these holes I'm not getting?
Good question! I don't think there's a specific unintuitive trick around the holes. The walls that pop up work the same as the holes (the only difference being a wall is created rather than a hole) so if you can solve level 4 you basically know everything you need to know about holes.
Gah.. I can't even see how to start 18 without going over the hole, and even then I'd need to steer each 'key' symbol over that point to get them to their 'keyholes'.. EDIT: Ok, of course, I'm forgetting all that I've been learning!.. Still stuck with how to get all the pieces in the required end position though!..