1. Launch the game once through Steam and configure all your settings how you like them to be, then create a save file.

2. Close the game.

3. Open the following directory in File Explorer:
C:\Users\{user}\AppData\Roaming\kingdom_rush_alliance

4. Open the slot1.lua (or whichever slot you created it at) file with a text editor (notepad++ recommended).

5. Search for "difficulty" in the file.

6. Edit the value to 4 (should be 2 on a brand new save).

Should look like this:
["difficulty"] = 4;

7. Enjoy playing the actual game since the start and not being locked behind a dumb wall for casuals to not cry about how the game is too hard.