Mana Shield

 is one of the final upgrades in the Magic Monkey Knowledge tree, along with Tiny Tornadoes and There Can Be Only One. When activated, a 'shield' will surround the player's life count, allowing it to absorb up to 25 lives before depletion. At the start of every round, 5 lives will be regenerated to that amount if no lives are lost last round, up to a max of 25. It requires eight knowledge points invested, Arcane Impale, and a one-time purchase of 1,000.

Summary
For all games where Monkey Knowledge is enabled, Mana Shield is incredibly useful for allowing the player to have an easier time surviving the early game, especially on harder game modes and harder maps, with exception of Impoppable Mode, where it is disabled. Mana Shield rewards players for saving lives, so it is important to avoid attempting to intentionally let lives leak.

Tips

 * Do not be afraid to let the shield tank some of the leaking bloons, with a maximum of 25 hit points, it can absorb a Lead Bloon entirely, with 2 lives remaining.
 * This upgrade is very powerful early on and taking just 5 rounds to recharge means it is very useful for soaking up any weaker bloons that may slip by.
 * Mana Shield allows all Camo Bloons on Easy Difficulty to be leaked, leaving behind 26 lives.
 * Lych's life-draining mechanic also eats up Mana Shield, so Mana Shield can be used as a clutch for lives.
 * Losing Mana Shield lives does not trigger Heart of Vengeance. However, it does not prevent triggering passive abilities from Elite Defender or Bomb Blitz.

Trivia

 * If playing on Easy difficulty with maximum lives and the shield is at maximum health, it is possible to tank 2 Ceramic Bloons, surviving with 17 lives.
 * Prior to Version 16.0, Mana Shield was considered a very powerful upgrade for Impoppable, as one could gain up to +25 regenerating lives in a single game. This was no longer the case as of Version 16.0, as it now disables Mana Shield.