Gasoline is the easiest level due to the small open nature it has, but Trainyard is very large and thus has many zombie spawns, leading to the max number of zombies spawning very quickly, on top of that, the spawns are scattered around the map leading to zombies spawning in front of you more often. Trainyard also is relatively closed off due to the train carts being scattered around the level. None of these issues happen on Gasoline, as it is arguably more open than Trainyard, and smaller in nature leading to the zombies being more controllable. It’s also incredibly easy to set up shop with a turret and camp in the corner of the level due to the low amount of zombies in the level, and this generally isn’t possible on Trainyard.
The motel boss level is also easier on Gasoline, due to it sharing similar characteristics with the normal gas station level (open, small, controllable zombie spawns) But the Trainyard boss level on a basketball court is more difficult due to it having narrow lanes on the sides + top of the level, and the court being closed off and only accessible through choke points that will be inaccessable if acid spit on / webbed off.
Trainyard is still the easiest because there’s that one spot you can shut out all zombies by just placing down a trap and going afk until it runs out. Gasoline’s medium because there’s nowhere good to camp you just have to deal with the zombies in an honest-to-goodness duel. Compound’s hard because the tight paths filled with props you get stuck on often get you killed, as well as having camping spots that are more dangerous than just taking on the enemies directly.
Common Cold
What is now considered Hard, but enemies are slightly slower and vampires deal less damage. 1x Units
Lethal Infection
What is now considered hard. No Changes. 1.1x Units.
Outbreak
Zombies are slightly faster, and there’s more of them. Special Enemies are more common and waves are longer. 1.25x Units
Pandemic
Upgrades are more expensive, Special enemies are even more common and deal more damage (Maybe some more special enemies can be introduced for higher difficulties?) Zombies will be more organised and come in larger groups. 1.35x Units
End of the World
The same increases as earlier but more of them, Cooldowns on Special items are longer, combo powers need more kills to activate, enemies will try to overwhelm you from multiple directions, Guns have less ammo. You’re not meant to survive this. 1.5x Units