I added a couple things to the beta this morning. If you want to try out the beta, right-click "Paradox Vector > Properties > Betas" and select "beta" from the drop down.
Updated Checkpoints
Some of the monsters were not removed after reloading a checkpoint. Now it should work with all the monsters.
Normal and Hard Mode
Currently the game will start in normal mode. This reduces the amount of damage monsters do to the player. It will also not spawn all the monsters in a given level.
Once the campaign is completed, it will unlock "Hard Mode" and those monsters will appear in the game.
Next Steps
I will be adding Steam Achievements soon. I will also be adding a menu option to select the difficulty level.