Hey, this game is kind of confusing for most people, (sorry) so here's some helpful instructions.
You can move with Left and Right arrows. Moving on to a space with an item will pick it up. This will replace whichever item you have currently in the square at the bottom (this item is the equipped item). You can change the equipped item with A and D or drop it with W. You can use the equipped item's effect with S. For example, the first item you get will always be the gohei (the red stick with white flail), which you can use with S to damage enemies. Item descriptions will appear near the bottom of the window. To refill your health you can "charge up" with the Up or Down arrows, if you press that key five times in a row, an empty heart will be filled. This is necessary to beat the lower levels. Empty hearts and cursed hearts cannot be used as inventory space.
The goal of this game is simple, kill enemies until you find a key, then use that key on the lock at the end of the level and move on through the red gateway (tori) at the end. Killing enemies with the gohei or shuriken will leave an item behind on some occasions. This can be any of the usable items in the game or a key. Enemies cannot pass through komainu/shiisa statues (the red and white dog things at the beginning of each level). There is a pagoda/lantern in the center of each level that will spawn enemies when it is onscreen. This will ensure you have a source of key-drops the entire game.
Hopefully this helps a bit, and hopefully this is legal in the terms of the jam.
Thanks for playing!