Health and Damage

This is a VERY HUGE mechanic that was introduced by Meta07. You can see health in BTD6 by hovering over the tower before you click it. The color of the line around it will decide health.
 * 100%-81%: White
 * 80%-56%: Green
 * 55%-26%: Yellow
 * 25%-1%: Red

In BTD6
Unless otherwise stated in the conception's page:
 * 1) The Angry Squirrel, Peltast Weasel, Watermelon Spitter, Meerkat Spy (due to their small size), and normal Mad Snowman (due to what it is made of) have 6 HP.
 * 2) Monkey Engineer constructs and Monkeys all have 10 HP. (e.g. Dart Monkeys, Ninja Monkeys, Monkey Engineer)
 * 3) Small machines (e.g. Bomb Towers, Tack Shooters, Bloonchippers, Non-Engineer Sentry Guns), Beekeepers, and Ice Towers have 15 HP.
 * 4) Vehicles have 20 HP (e.g. Monkey Aces, Monkey Buccaneers).
 * 5) Buildings and armored monkeys (e.g. Mortar Towers, Dartling Guns, Super Monkeys, Monkey Villages, Hospitals) have 40 HP.
 * 6) Large, armored vehicles, Mad Mastodons, and pro Mad Snowmen have 60HP (e.g. Monkey Tanks, Destroyers, Spectres)
 * 7) Giant Buildings (e.g. Temples of the Monkey God, Bloonsday Devices (not Micro!), Monkey Towns, M.I.B.s, and Banana Research Facilities) have 100 HP.
 * 8) Fortresses (the Monkey Battleship and the Omegachi) have 1000 HP and only appear in special missions.
 * 9) Terrain Agents have only 1 HP.

Immunities and Weaknesses

 * Dispense-o-matics and sentry guns takes a boatload more damage from projectiles and explosives. (x20)
 * Monkey Engineers are immune to projectile weapons, but this still will (almost always) make the same damage. (x.95)
 * Monkey Aces that are hit by energy and plasma-based weapons suffer more damage that other vehicles. (x1.5)
 * Otaku Shelters and Ice Towers take less damage from explosives. (x.5)
 * Danmaku Monkeys suffer less damage from energy projectiles. (x.5)
 * Minecraft Monkeys are protected from anything, depending on armor type.
 * Leather: x.7
 * Gold: x.6
 * Chain: x.5
 * Iron: x.4
 * Diamond: x.25
 * (x/3 has iron armor)
 * Banana Farms take way more damage than expected near explosions and fire-type projectiles. (x2)
 * Ninja Monkeys are immune to magical weapons and ninja weapons. (x.625)
 * Sword Monkeys and Monkey Archers are protected from any projectile, depending on upgrade. (x.75 to x.25)
 * Normal Mad Snowmen take a massive amount of damage from fire-type projectiles and explosions. (x15)
 * Monkey Apprentices takes less damage from magical projectiles and weapons. If made a Disciple of Air or Fire, this will increase strength. (x.7 to x.4)
 * ToTMG's are resistant to everything! (x.5)
 * If you ACTUALLY get a Satanic Temple, the bloons will meet their doom. More resistant to all the things! (x.1)
 * Beekeepers and Mad Mastodons can't be stunned or slowed down.
 * Terrain Agents, although they only have 1 HP, can't take ANY damage WHAT-SO-EVER. (x0)
 * Pro Mad Snowmen take WAY less damage than expected from heat-based attacks (including explosions) (you expected x15, you got x.5)
 * Monkey Medics using the Medi Gun's Ubercharge, along with their patients cannot be damaged by anything. (x0)
 * Monkey Soldiers equipped with a Battalion's Backup and all surrounding towers within 200px radius take x0.2 damage when they sound the horn. (x0.2)
 * Sentry Guns that are being wrangled by nearby Monkey Engineers take less than half the normal damage. (x0.25)

Evade Chances

 * Buildings and some robotic towers can't move at all... (0% Chance)
 * Flying towers are fast enough to evade many attacks. (62% Chance)
 * Stealthy towers (Ninja Monkey, Danmaku Monkey, Stealth Monkey etc.) are stealthy enough to evade most attacks. (56% Chance)
 * Super Monkeys can evade some attacks due to their flexibility. (32% Chance)
 * Clones, shadows and holograms evade every, they are always first priority. (100% Chance)
 * Terrain Agents can't be aimed at all! (100% Chance)
 * Towers placed underwater (i.e. Monkey Submarine) can easily evade some attacks. (30% Chance)
 * The Cosmic Monolith can warp Spacetime, allowing it to evade pretty much anything. It can even evade explosions it is in the middle of! (85% to 94% chance)
 * Continuous laser attacks (V.A.M.P.I.R.E. Vampirism, etc.) can not be easily evaded, but the chances are "refreshed" every second (i.e. If a tower is hit by a laser that lasts 3 secs, it has a chance to evade at the 1st second, which means that it will completely evade the laser, but if the tower does not evades at the 1st second, it can evade at the 2nd or 3rd second). (17% Chance)
 * Explosive attacks can't be dodged, unless the tower is currently at the BORDER of the AoE (Area of Effect).

Bloon Bosses

 * Bloon Bosses (like Apopalypse) will have an HP bar. Also, they will get wounded just like towers. Their wounds aren't red because bloons don't have blood. An example: Apopalypse_HP.png

In Bloons III

 * In Bloons III, The Monkey has 10 HP at the start, but can buy armors to increase his HP.

Absolute Defense

 * Some heavily-armored towers have Absolute Defense (AD). Each AD point means that when that tower gets damaged, the tower will receive one HP less damage in damage calculation (for example, a tower with 5 AD will only lose 5 HP (10-5) when get hit by a 10 HP attack), which also means it'll gain complete immunity against certain attacks (mostly machineguns).
 * Some attacks can penetrate armor (most likely needle-shaped projectiles), which means they can ignore some points of AD. For example, a tower with 5 AD will still lose 10 HP when get hit by a 10 HP-but-can-penetrate-5-AD attack.
 * Some attacks, mostly wave attacks, can completely ignore AD. However, these attacks can be weaken by wave jammers, or armor with built-in wave jammer. :P
 * Armor penetration can also come in the form of multipliers. For example, a 2x-penetration-attack will divide the target's AD by 2 (rounded down) upon damage calculation. If this certain attack can deal 10 HP damage, and hits a tower with 5 AD, then that tower will lose 8 HP (5/2 = 2.5, rounded down = 2, so the tower's effective AD is now 2. That means the tower gets 10-2=8 HP damage).
 * Some attacks are even specially designed to break shields, making the target lose some of its AD points permanently!

In Bloons Super Monkey 3

 * The Super Monkey starts with 100 HP, and can buy armours to increase his HP, just like the monkey in Bloons 3.
 * You can also craft some armours (like RegMat armour).

Trivia

 * Damage is is rounded to the nearest whole number. (BTD6)
 * 4/4 towers all have 500 HP. If the 4/4 tower is destroyed, the mission for the tower is over. (BTD6)
 * If you find the Satanic Temple Easter Egg, you'll have a tower with 1,000 HP. (BTD6)