As you probably already know, if you stand on the spawning point, you immediately die. You can stand there before anything spawns and looks like the game just constantly restarts. For example, I keep jumping on to the second platform before anything spawns and am taken back to the start screen without seeing any zombies.
However, this means you got the basic loop of collisions and death room return down.
It's weird to use the mouse to start the game, but have the keyboard as the game controls; however, I'm guessing you must have pushed yourself to try to handle different types of user input. Nice.
The sprites also properly hit the walls, stand on platforms, and change facing direction.
The sprites are also very detailed, so if you drew all of that from scratch, great job!
Also, music, great job if you made it from scratch! Getting it to play, also great job.
If this is your first game, and you are new to programming logic, I think you must have grown a lot with this project. :) Also, great for a solo dev.