Really detailed for the amount of time we had! Three levels and multiple enemy types and three songs is really impressive! The basics of the game feel really clean and smooth, it's really really solid all around once you get the hang of it. The difficulty curve is a little steep but it helps you pick up the game in the short amount of time we get with it.
I think the heartbeat concept kind of hinders the gameplay a little though. In games like Hollow Knight, there are already times where you're incentivized or disincentivized to attack by the enemy attack patterns or arena. For example when your flying boss is in the air or when you have attacks thrown at you. So filtering it through that extra level of "is my heart about to beat" is a bit awkward and it's really hard to focus on both at once. Hollow Knight x Crypt of the Necrodancer sounds like a dream and I see the vision, but I think it could have used some extra time to really blend those concepts together seamlessly.