There is a door. Look around you the next time you reach level 15

I find 26 easier than 11 too (though I have the

)... The level with an apple, a melon and a strawberry is a bit harder than the PD26 levels

*hehe* Yes, PD13 is being neglected... But I would go through 11 anyway because it is far easier than 13
level 82.1.x is missing - there is this door that requires the "Inversion key", which does not exist at all. Aside from that, there is no known PD missing (okay, the few levels to the Snow bomb are not listed)