When the game starts, you see the level assets appear one by one. I think that might be what causes falling through the scenery. My PC has relatively high specs, so it loads it fast enough, but if it was slower, the player would probably experience what Was A Horse experienced.
The first boss doesn't take damage, or at least, I didn't find a way to hurt him.
Your game's scope is quite big, and I aplaud you for what you've done so far. Thanks for the experience.