A slightly earlier version of the source code, that is built with Pygame Zero is available here. The version that is running on browser, and the Windows download available are the ported version to Pygame, with minimal changes. 

How to play?

•     WASD to move, SPACEBAR to shoot. 
•     Kill the incoming enemies, and collect the dropped xp orbs.
•     Choose an upgrade each time you level up.
•     Fill your ubercharge meter by killing enemies.
•     When ubercharge is active, you are unkillable and get max upgrades for all stats.
•     Reach level 40 as fast as possible to win.
•     There is a difficulty setting in the options menu. 

Public assets used:

•     "16x Tileset, Mostly Flowers" by ArkyonVeil
•     "Oleaguid Font" by arynoc
•     "Set of pixel art projectiles" by davidaf3
•     "Cat Fighter Addon2[ Assault Rifle Kit ]", "Cute Monster Sprite Sheet" and "Skull Monster Sprite Sheet" by dogchicken 
•     "RPG GUI Selection Arrow" by INFECTiON656
•     "Grass Tiles" by Invincible
•     "15 monster grunt/pain/death sounds" by Michel Baradari
•     "Futuristic SMG" by Michael Klier
•    "75 CC0 breaking / falling / hit sfx" by rubberduck
•    "Osmotic Memory" and "Echoes of Yesterday" by Tsorthan Grove
•     "Win and lose melodies" by Vircon32

NOTE: This is a demo, and for testing purposes, you can press the "O" key to gain a level and finish the game.