Tier Unlocking

Tier Unlocking is a feature in BTD6. For a modest sum, you can enable the ability to get both Tier 3, Tier 4, and even Tier 5 upgrades on a single tower. These upgrades are all substantially stronger than an equivalent 2/2 upgrade. Unlocking the 4th and 5th tiers often require a special Crafted Item along with money. The names given are not the name of the upgrade but the name of the tower.

Dart Monkey
Triple Spike-o-Pult: Exactly like the Multi Shot upgrade for the Spike-o-Pult in BTD3 and the Dart Monkey in BTD4. Hurls three spiked balls at once, with each acting like a normal one. Costs 200 Monkey Money to unlock.

Tack Shooter
Blade Sprayer: Shoots 16 blades at a time. Costs 225 Monkey Money to unlock.

Sniper Monkey
Deadly Semi-Auto Sniper: Can break through an entire Ceramic bloon in one shot and fires 3 times as fast as an x/2 Sniper. Costs 300 Monkey Money to unlock.

Boomerang Thrower
Bionic Ricochet: Throws glaives at supersonic speeds. These glaives bounce from bloon to bloon. Costs 400 Monkey Money to unlock.

Ninja Monkey
Double Flash Bomb Ninja: Throws two shurikens and two flash bombs. Smoke bombs can pop frozen and lead bloons and stop bloons for a short time. Costs 500 Monkey Money to unlock.

Bomb Tower
Cluster Mauler: Shoots modified missiles that deal 10x damage to MOAB-class bloons. Sends off smaller explosives that deal 5x as much damage to MOAB-class bloons. Costs 600 Monkey Money to unlock.

Tier 4s
All Tier 4s require having that tower's 3/3 upgrade unlocked previously.

Dart Monkey
4/3: Triple Juggernaut: Like the Triple Spike-o-pult, only with the powerful Juggernaut projectile. Costs 400 Monkey Money and an unspecified item to unlock.

3/4: Super Monkey Fan Spike-o-Pult: Like the Triple Spike-o-pult, but it can be affected by the Super Monkey Fan Club ability and can activate it. Costs 400 Monkey Money and an unspecified item to unlock.

4/4: Super Monkey Fan Juggernaut: Like the 3/4 upgrade, but a Juggernaut. Costs 600 Monkey Money and the 4/3 and 3/4 upgrades already unlocked to unlock.

Tack Shooter
4/3: Blade Blaster: Shoots 32 blades at a time. Costs 450 Monkey Money and an unspecified item to unlock.

3/4: Super Blade Sprayers: Shoots 16 super-strong blades at a time. Costs 450 Monkey Money and an unspecified item to unlock.

4/4: Super Blade Blaster: Shoots 32 super-strong blades at a time. Costs 675 Monkey Money and the 4/3 and 3/4 upgrades already unlocked to unlock.

Sniper Monkey
4/3: Semi-automatic Cripple MOAB: Shoots three times as fast and temporarily halts MOAB-class bloons. Costs 600 Monkey Money and an unspecified item to unlock.

3/4: Deadly Full-auto: Shoots extremely fast and can destroy a Ceramic in one hit. Costs 600 Monkey Money and an unspecified item to unlock.

4/4: MOAB-crippling Full-auto: Shoots extremely fast and temporarily halts MOAB-class bloons. Costs 900 Monkey Money and the 4/3 and 3/4 upgrades already unlocked to unlock.

Boomerang Thrower
4/3: Bionic Master: TBA. Costs 800 Monkey Money and an unspecified item to unlock.

3/4: Double Bionic Ricochet: Throws two ricochetting glaives at supersonic speeds. Costs 800 Monkey Money and an unspecified item to unlock.

4/4: Double Bionic Master: TBA. Costs 1200 Monkey Money and the 4/3 and 3/4 upgrades already unlocked to unlock.

Ninja Monkey
4/3: Flash Bomb Bloonjitsu: Throws 5 shurikens and 5 flash bombs. Costs 1000 Monkey Money and an unspecified item to unlock.

3/4: Double Smoke Bomb Ninja: Throws 2 shurikens, 2 flash bombs, and 2 smoke bombs. Smoke bombs have a 5% chance each of removing camo and regen properties. Like the Smoke Bomb upgrade, gives the ninja an ability that lets it move one monkey tower using smoke bombs. Costs 1000 Monkey Money and an unspecified item to unlock.

4/4: Smoke Bomb Bloonjitsu: Throws 5 shurikens, 5 flash bombs, and 5 smoke bombs. Costs 1500 Monkey Money and the 4/3 and 3/4 upgrades already unlocked to unlock.

Bomb Tower
4/3: MOAB Impact: Shoots powerful, violent missiles that deal 10x damage to MOAB-class bloons and send off smaller explosives that deal 5x damage; the explosions can pop explosion-resistant bloons and stun all bloons for a short time. Costs 1200 Monkey Money and an unspecified item to unlock.

3/4: Cluster Assassin: Very similar to the Cluster Mauler, except it gets the MOAB Assassin ability and deals 12.5x damage to MOAB-class bloons with its main explosions. Costs 1200 Monkey Money and an unspecified item to unlock.

4/4: Assassin Impact: Combines the power of the MOAB Impact and the Cluster Assassin into one monstrous tower. Costs 1800 Monkey Money and the 4/3 and 3/4 upgrades already unlocked to unlock.