Have you ever encountered an enemy that looks... a little different than usual?
Say, a Hedgehogger wearing a football helmet?
Where many games use color changes, in 19, enemies wear costumes to indicate their strength.
The football helmet, for example, is the level 1 version of the Hedgehogger (the costume-less version is called level 0). An enemy's level determines how much damage you take if it hits you.
Normally, it is up to the player to figure out how much damage specific enemies do, but this time, it's on the house; a level 0 Hedgehogger does 2 damage, but a level 1 version does 1 extra damage on top of that for a total of 3.
Can you guess how much damage a level 2 Hedgehogger does? And a level 3? Hint: you can use the same math that gets you from level 0 to level 1.
Including level 0, most enemies have 4 levels in total, with each level being slightly stronger than the last. It's important to keep in mind that you will never encounter a level 2 before a level 1; you will always see the levels of any enemy in ascending order so it is easy to remember which is which.