We had a few challenges:

• Since all transactions on the blockchain are public, we had to figure out a way to let users commit to a 'hand' in a game without the ability to (1) change it afterwards, (2) back out of the game once they see the other player's hand. We solved this by implementing a commit-reveal scheme where users first commit to a game by submitting a hash of their hand + a secret phrase, and once both users have commited they will have to both 'reveal' by publicising the secret phrase and hand - which is then checked against their commit on-chain.
• We had to make sure to make the game engaging enough, even with the cumbersome UX of a commit-reveal scheme. We worked really hard to refine our frontend to make it animated, easy to parse, and generally fun to use. Its hard to say if we solved this problem or not, but we are pretty proud of what we made.
• There aren't any good indexing services that allow developers to listen to smart contract events in an easy and reliable manner. To solve this problem, we had to build our own block indexer that uses Coinbase's managed node solution.