You can't jump and then move left you have to be moving left and then jump. Hit detection is terrible where floating platforms are actually rectangles with invisible portions on the ends. The gun always shoots to the right, always! Bullets sometimes randomly stray onto the screen and just kind of hang there in the air levitating forever. You stick onto the sides of walls by constantly holding that directional key.
The zombies just sit there which I guess is better than them doing anything else with how broken the controls and hit detection are in this game.
The black flag is not the end of level marker but you have to kill/shoot it and then move on as touching it just restarts the game. Mean trick. You have to click the gold circle at the upper-right at the boss screen to "Finish the Game". Also mean trick.