Start screen ui is not the best, since you block the player sprite a lot. Fix for readable font and more simpler UI.
Biggest Issue
I see you have the instructions in the page, but most of this should be in your game. The main issue with this is that people should not have to read somewhere else to play your game because it 1) makes people confused due to the lack of information and 2) wastes time which decreases player experience. You want people to play your game, not read how to play it.
UI
Overall, not good. Clearly indicate the HP and Level in the game. Clearly indicate the description for the items in the game. Add in UI for showing the player they levelled up and won the battle.
Indicate [Area1, Area2] for different areas. It can be a small animation on the top right corner to indicate the player is progressing.
Indicate the description and base damage of Slash and Fire Sword and Call Army in the UI box.
Magic Sword does not decrease magic.
Indicate when the player receives a new item. I did not realize I had a new one until I checked my items.
SFX
Battle: There arent any sound effects for battle music, attacking, pressing an input, or going back in the battle menu.
World: There isnt any music for each area and walking. It could be nice to have the enemies have a cry or some sound they make when you are nearby. You can show progression for each area by making it more tense, or whatever feeling you would like to convey in your game.
Art
UI and background art is very basic. Character art and basic enemies are okay. Boss should not have the same colored sprite as the player. Walking animation would be good.
Battling
Army mechanic is not clear in the game and it should be. I think vfx for the moves could be a nice addition. Most of the battling issues stem from a poor UI design where things need to be obvious and not hidden.
Narrative
Difficult to tell why the player starts off battling. There is a spare mechanic not mentioned in the game, so it would be useful to outline these things in the game. You can use the first battle to mention how to play properly.
Overall the game experience is not too good because of the issues I've mentioned above. I think you should look into game design and playtesting to improve. You need to let players think about what they are doing for turn based combat. It needs to generally be fun and fresh when you battle with the exception of grinding. There is not a clear direction of what the player is fighting for necessarily, and adding a story can help with that. The army mechanic is interesting, but I am not entirely sure if the damage scales based on how many people were spared since there is no description of how it works in-game. The army mechanic is creative but needs to be worked on. However, the game is poor in UI, art, SFX/music and narrative.