Nuclear Throne has replaced Spelunky as my go-to "just one more try" game. I decided to check out some videos of what lies beyond the third world, and the answer is... SO MANY BULLETS.
Vlambeer recently added a 7th world, the Palace, and it looks like instant death. I haven't even gotten close to it, so enjoy this video where YouTuber Dishonourable Daimyo does an excellent run to the palace with Y.V. and calmly narrates with his lovely accent.
Let's Play Nuclear Throne 19 [YouTube]