My guess is that you didn't grab the key! Some levels require you to do certain things for the doors to open, and it's not really communicated to the player. The playtests I've watched have involved a lot of going "hm..." and then exploring haha. Thanks for playing and the feedback! I'll try to get some VFX in there heheh.
P.S The spin does in fact start slowly on purpose! The damage you do is proportional to the speed at which you're spinning, so faster spinning = more damage, but you have to sacrifice your stamina to do so (which results in less capacity to sprint/avoid attacks). And like you said it requires forethought before engaging with enemies.