You can see the Bird Cage for yourself by downloading it, and dropping it under your "saves" file.
(NOTE: You must have Minecraft 1.8.9 or higher to use this world without worry of corruption!).
Link to download: http://www.mediafire.com/download/eejpcposuia0qss/Flying+Gym.zip
Images are located here: http://imgur.com/a/Rhjrb
NOTE: This gym is 35x30 due to the protruding entrance. Luckily, there is a hefty amount of air in this build, so removing that should make implementation easier (and doesn't change anything ingame).
If the minor extrusion is really a big deal, I can gladly cut it. It just may not look as good, keep that in mind.
After beating the gym leader at the very top, the player breaks the Sea Lantern in the middle of the pokeball and falls all the way down, landing safely in the water below.
The idea is that the trainers will be lurking in the branches. You need to jump from branch to branch in order to get to the top. Beware of the height. Soar high or fall and die...
The path to the gym leader is 100% accessible, albeit some branches being extremely difficult to jump, requiring good timing.
I did not add the trainers, gym leader, or the fancy door. I do not work with NPCs or redstone/command blocks. However, I did make the entrance bulky enough so that redstone could be sneaked in. If someone could do that, please, that would be splendid.
UPDATE: Newer pictures here. The Dirt block at the top was replaced with a Sea Lantern.
Model Rater, Pixelmon Tester, Official Forum Supporter, Gym Development Member!
Not the Pokemon we need, but the one we deserve.