Lives or a Life (singular), is the main factor used by the game to determine how well the player has beaten the level. Located in the top corner of the screen next to the Gold counter, the player starts a standard Campaign level with 20 lives. Iron Challenge and Heroic Challenge modes give the layer only 1 single life, whilst the Endless Challenge gives the player 10 lives.
- Lives are lost when enemies cross the exit. Enemies can cost between one and six lives, depending on how strong they are (usually, but not always). Fortunately, they cost the same amount of lives regardless of what difficulty the game is being played on. They also give the player their standard bounty.
- Bosses and mini-bosses from the Endless Challenge cost 20 lives, therefore if a Boss crosses the exit, the player loses all of their lives and automatically loses the level.
- Completing a Campaign level awards stars based on the number of lives left:
- 18+ lives: 3 stars
- 6-17 lives: 2 stars
- 1-5 lives: 1 star
- In Kingdom Rush and Kingdom Rush: Frontiers, players on mobile platforms can purchase a Heart Box from the Shop. The Heart Box can be used in a level to provide an additional five lives. As many heart boxes can be used as wanted, providing whatever amount of lives the player wants.
- In Kingdom Rush: Vengeance there is Hearts in a Bottle in Shop that increases 3 lives.
- Heart Box is very useful in the Endless Stages, and tend to be most worthwile in very late waves of the game. Minibosses tend to have very high health, starting at 70, and your towers won't be able to deal that much damage and kill the Miniboss. However, since Miniboss costs 20 hearts, you can use 4 Heart Boxes And let the Miniboss pass the stage. That way, you save a lot of trouble and your other items as well.
- A Heart Box used in either the Heroic or Iron challenges will boost your available lives up past a single 1, giving you a better chance of surviving if you are having difficulty with enemies leaking through your defenses.
- Likewise, using one on a boss level will give you more than 20 lives (assuming you haven't lost any) allowing you to survive a Boss crossing the line. This will result in a victory for you, but with only a low star reward.
- In Kingdom Rush: Vengeance, however, bosses cost a whopping 999 lives, meaning to increase the difficulty of boss levels, preventing the player from letting the boss cross the line.
- Due to the changed items, there is no way to increase your lives in Kingdom Rush: Origins.