This is a really cool concept. I like the art and the simplicity in the game and would love to see continued development. I think it would be cool if it were a bit faster (though maybe this is due to my phone? I tested it on a Galaxy S5 so I'm assuming there shouldn't be any slowdown). I too had some difficulty understanding the delta time thing since there's nothing that mentions it anywhere that I could find. The game also doesn't seem to have an exit button.
The controls feel really good when shooting as well, though since there doesn't seem to be anything else the player would need to interact with aside from their own archer, I think it may be better if the player could hold and drag anywhere on the screen to control the angle/trajectory of the arrows. That way they aren't always having to reach over to the archer to shoot, and can more easily see where and how far they're shooting, especially given that this is all happening in real time and you have to be conscious of where you're standing when arrows are coming at you.
That said, this is really cool and I hope you keep adding to it!