I really liked this demo for Fiend's Isle. I liked the aesthetic of the game with the PS1 style going for it as well as the game mechanics itself. There's really isn't a tutorial on how to fight enemies or what the different kinds of items there are in the game, which gives you a hit of dopamine every time you discover something new. I went through the game without knowing that there was a healing mechanic that could have prevented me from dying. I didn't know that you could pick up the patches of grass on the ground and use them to heal yourself, so I kept running through areas with low hp and surviving with my top tier dodging skills.
There are a few issues I had with the game mechanics that kind of got in the way of my enjoyment of the game. The first was with the range of the attacks. Sometimes, I would swing at a slime or skeleton with my dagger, and it would miss so I would try coming closer to guarantee a hit and I would end up taking damage. Also, without some kind of crosshair marking where you're clicking, there is no sense of knowing where the attack is going to land. Therefore, I think adding a crosshair or a dot to mark the middle of the screen along with adding just a tiny bit of range to the weapons would help fix these problems.
Another issue I faced while playing is the UI of the inventory. I thought I could shoot a skeleton from far away with my crossbow and the switch over to my short sword to finish it off, but I got stuck in the middle of swapping and ended up dying because of it. To make it easier to swap weapons in the middle of combat, I would suggest making the scroll wheel bigger to display more items, since it only displays two at a time, or to make a section dedicated to holding your items and have the display screen for the items to the side.
Overall, I found his to be a pretty fun demo. I understand that this isn't the complete game and that it isn't the finished product. The problems I listed are pretty small and don't do a lot to take away from the enjoyment of the game. If you made it to the end of this to figure out if this is a game worth playing, I think you should try it out.
TLDR: Fun game with a few problems