The fundamental elements of a Zelda-like action adventure game are here - the character movement is very responsive and feels good, the enemies rush at you with just the right speed, and there is some satisfying visual feedback when you score a hit and fun enemy death animations. Progress is made by collecting keys from dead enemies (except 1, found in another way) to unlock gates to eventually reach the stairs at the end. This is a classic game loop and the developer succeeds on a lot the major necessities.
The game is hampered by a lack of balance, which is a constant factor as there is only one enemy type. Your attacks are limited by a stamina bar which depletes after about 3 swings (enemies take two hits to kill), which would lead to cautious attack-and-retreat gameplay if the enemies were any threat. Unfortunately, there is a delay on their attack animation that only triggers 2 or 3 seconds after they collide with you, making it impossible to die unless you take your hands off the keyboard. My suggestion would be to remove the stamina system entirely (or drastically ramp up the amount you have/regenerate) and make the enemies much more dangerous - maybe a single heart of damage on initial collision and two hearts after they breath fire on you.
That being said, if the balance were fixed and a couple of the key doors were replaced with puzzles of some kind, this would be a completely playable and enjoyable game. As is, it's a solid prototype and a great first effort for the developer. Keep it up!