You can't enter the Birdland, the only entrance to a secret level that was finished was the one in Medieval Kineval to Gargoyle's Lair. There's also one in Voltage Village near the end, but it's entirely sealed off. All of the other secret levels would have been useless at this point anyway. Return to Birdland can't really work gameplay-wise because the bird can't shoot. There's also a Psych secret level, but it's unfinished (it was finished by someone else a couple of years back, try searching the J2O database for "Retro Wasteland"). The Diamondus secret level is nothing more than a night version of the first level, and the Tubelectric one isn't even started on! You can open all of them in JCS for a quick peek.

- JelZe GoldRabbit =:3