There needs to be some sort of hint about that imo, because I would have never guessed that cutting grass opens a door. Or like a grass tile with a different image like the cracks in zelda.
The final game will come with an instructions manual that explains the mechanic. In any case, we will most likely add an explanation in-game. You aren't the only one who had trouble finding the cave.
Now that I hacked every bit of grass in the game and got into the dungeon, I was able to beat the demo. The dungeon was fine, my big beef was that the boss of the demo was way too difficult for me. I may be a filthy casual, but it took me about 10 tries to beat him and learn enough about the chaotic pattern he moves in. It still seems sort of random but usually when he does the loop move he does it twice in a row and then is a bit easier to hit a couple of times. I couldn't handle how unpredictable he was so I save state scummed my way through it. If I had been honest I would have had to beat the dungeon at least 10 times to beat the boss which is too much imo. In zelda you usually restart at the beginning of the dungeon, it needs to be easier to try again with the boss.
Great work though, no technical glitches at all, and the game was polished and nintendo quality. Difficulty was WAY up there but great demo. If I were a publisher I would offer to publish this game but I have no ability there. Please keep me posted with updates so I can play new versions.
Thank you for playing and for the feedback.

The boss can be difficult. The trick is that he only whirls to the right. Fighting him from the top can also be effective.
The best strategy is having 2-3 apples to recover HP, and just cheezin him. Did you had any apples when you fought him?
The game has a checkpoint feature close to the boss. You must step on the 'flashing triangle' inside the cave to 'save' your progress up to that point. To access the triangle you need to drop from one of the holes in the second floor.