From a purely engineering perspective, it's been the networking. It turns out making a physics oriented game with 60 players is actually quite an undertaking!
Every level / obstacle has been a case of figuring out exactly how efficiently we can relay information to clients to maintain good server performance / networking throughput whilst providing a fun and responsive user experience.
Levels are often iterated on multiple times to improve performance over the course of their lifetime, and finding the perfect balance between design and engineering has been a fun and interesting challenge!
Some levels have been much more difficult than others (I'm looking at you Egg Scramble 😠 ) But it's been a fantastic challenge and we are constantly improving the experience however we can!