I am delighted to hear you have finished the game. Glad you like it.
And thank you for the great feedback :D It really helps to discover flaws in the level-design/ introductory style that I have tried.
So these are my takeaways:
Since the barrel has to become slightly transparent (so you can see what is in front you), the first encounter should have been against only one enemy. That way you won't get hit from 2 different angles what essentially lead to your first experience. To the second point: I guess the best way would have been to make it somehow more obvious. To not let the player pass trough that door with the barrel in hand would be one or to introduce "blocking doors" before in a way you don't die if you miss it, would be the other.
And may I ask how the last fight against the 3 guys upstairs went? At first this was more like a puzzle as well. But then I got scared that it was way to hard and added more barrels (since there are no checkpoints and dying to often there would be very frustrating) which in the end weakened the puzzle ^^