Since it's a prototype, I'll just list possible problems:
– The first time around, 'space to jump' didn't work for me, until I started using a gamepad, then space jumping became possible, too (as you mention in the description)
– Don't use Dpad on gamepad for movement. The left stick is typically for movement. You can use it even now, although it doesn't turn you.
– I'd reconsider instant death from spikes. There's a lot of them around and with no checkpoints, it may become frustrating
– 'Zero' health still has a thin green line at the left when your health is 0. If you do this by setting localScale, try fill amount instead:
https://docs.unity3d.com/Packages/com.unity.ugui@1.0/api/UnityEngine.UI.Image.html#UnityEngine_UI_Image_fillAmount
– The jumping animation is super slow
– 'You died' & Restart button are pretty small
– There are different pixel art resolution being used (e.g. the main character vs the enemies)
– Enemies are blurry (bad image? downsized?). Try the pixel perfect package if it helps.
– The last part of the level is quite boring, it's a long predictable corridor making you charge the gun, shoot, move a few steps, charge the gun again several times in a row
– The boss doesn't do anything interesting, and if you didn't have a hiding spot at the entrance, he'd be impossible
– I think the gun has a pretty short range. But anyway, the blast should not just disappear, there should be a 'dispersing' animation
Good luck with further development.