The main difference between the difficulties is the health of enemies: enemies have less health on Casual and more health on Veteran, compared to the numbers displayed in the in-game encyclopedia. In the first Kingdom Rush the difference is always 20% less on Casual and 20% more on Veteran. The only exception is Vez'nan, who always has 6666 health in his demon form regardless of difficulty (his health in human form is still affected normally).
Difficulty also indirectly influences the enemies reanimated from graveyards. Enemies with less than 300 health are reanimated as Skeletons, and enemies with at least 300 health as Skeleton Knights, so:
- Dark Knights (240/300/360 HP) are reanimated as Skeletons on Casual mode, and as Skeleton Knights on Normal and Veteran mode.
- Demon Spawn (200/250/300 HP) are reanimated as Skeletons on Casual and Normal mode, but as Skeleton Knights on Veteran mode.
- Flash: Trolls (224/280/336 HP) are reanimated as Skeletons on Casual and Normal mode, but as Skeleton Knights on Veteran mode.
In addition, difficulty also affects the speed at which heroes gain experience: the XP gain is slower on higher difficulties. Because of this, in Kingdom Rush: Frontiers and Kingdom Rush: Origins, it is recommended to train heroes on Casual instead of Normal or Veteran.
The Achievements Great Defender, Heroic Defender and Iron Defender can only be obtained on Normal or Veteran. Supreme Defender can only be obtained on Veteran.
Kingdom Rush: Vengeance sees the addition of a fourth Difficulty level Impossible. Contrary to the meaning of the word "Impossible" itself, it is still possible to complete a level in this mode, but rather a real challenge for most players to handle. Impossible mode sets enemy HP to a higher level again.
In March 2019, an Impossible Mode was added to the Steam version of Kingdom Rush: Origins, which introduces new and unique mechanics. Instead of enemies merely gaining HP, they can get a variety of buffs: more health, more damage, better armor, more speed, or taking more lives. Every enemy gets different kinds of changes, with some becoming much more powerful, some getting a small buff, and a few remaining unchanged. Presumably, this intricate system had to be created since the difficulty was introduced retroactively, and the developers had to make sure it didn't make any level completely impossible. In any case, it certainly makes some levels and Challenges, especially on the Elite stages, extremely difficult.
GREAT DEFENDER Complete all Campaign stages in Normal difficulty.
HEROIC DEFENDER Complete all Heroic stages in Normal difficulty.
IRON DEFENDER Complete all Iron stages in Normal difficulty.
SUPREME DEFENDER Complete the campaign in Veteran mode.
GREAT DEFENDER Complete the Campaign on Normal difficulty.
HEROIC RUSH Complete all Heroic Challenges.
IRON RUSH Complete all Iron Challenges.
SUPREME DEFENDER Complete the Campaign on Veteran difficulty.
IRONBORN (30 gems) Win all the Iron mode challenges
FROM ZERO TO HERO (30 gems) Win all the Heroic mode challenges