Challenges

Challenges can be found in BTD4 and you have to beat a certain level under certain restrictions in order to win one. The challenges are:

Ocean Road
Difficulty: Medium

Objective: Beat this track on Medium using only Dart Monkeys and Super Monkeys. You can upgrade them to Spike-O-Pults and Sun Gods, respectively. It is quite difficult to destroy the Lead Bloons, unless you have the Exploding Darts Premium Upgrade or the Double Cash Mode enabled, as the Sun God is needed. Otherwise, you will have to rely on timing Pineapples at the right time.

Ant Hill
Difficulty: Hard

Objective: Beat this track with only one life.

Lollipop
Difficulty: Easy

Objective: Beat this track with only Dartling Guns and Spike Factories.

Pool Table
Difficulty: Medium

Objective: Defeat a single wave of 50 MOABS with $50,000.

Ocean Road
Difficulty: Easy

Objective: Same as above.

World Tour
Difficulty: Medium

Objective: Beat this track without using Glue Gunners or Ice Towers.

Military Base
Difficulty: Medium

Objective: Beat this track with Dart Monkeys and Tack Shooters disabled.

River Bed
Difficulty: Medium

Objective: Beat this track using any tower, but at most three of each kind.

Bus Route
Difficulty: Medium

Objective: Same as Ant Hill on the computer; if even a red bloon escapes, it's game over.

Cactus Creek
Difficulty: Hard

Objective: Beat this track using only Boomerang Throwers, Glue Gunners, and Mortar Towers.

Snow Trail
Difficulty: Hard

Objective: Beat this track using only the Monkey Apprentice, Ice Towers, and Bomb Towers.

Go Bananas
Difficulty: Medium

Objective: Deflation-mode challenge, where you start at round 40 and have to make it to round 85.

Blue Laser
Difficulty: Hard

Objective: No Super Monkeys or Banana Farms

Daisy Chain
Difficulty: Hard

Objective: Only Dartling Gunners and Pineapples

Pool Table
Difficulty: Medium

Objective: Same as Desktop version above except popping Bloons gives no cash

Storm Cloud
Difficulty: Hard

Objective: All Towers but no Level 4 Upgrades

Lava Lake
Difficulty: Apopalypse to round 45

Objective: Limited towers: Bomb Tower, Monkey Ace, Banana farm, Boomerang Thrower. $1000 Cash.

Ocean Road
Difficulty: Hard

Objective: Somewhat similar to Full House in BTD5, only 6 towers allowed at a time

Sweet Tooth
Difficulty: Easy

Objective: Similar to There Can Be Only One in BTD5, only 1 tower of each.

Firecracker
Difficulty: Hard

Objective: Same as above.

Triple 1
Difficulty: Go Bananas Medium to round 50

Unlocked at rank 20. Objective: 1 of each tower, 1 life, and only 1 upgrade allowed.

Pirate Curse
Difficulty: Jolly Roger Medium to round 50

Unlocked at rank 30. Objective: All towers allowed but only 2 upgrades allowed.

Financial Crisis
Difficulty: World Tour Hard to round 60

Unlcoked at rank 50. Objective: No Banana Farms and cash earned is halved.

Supply Lines
Difficulty: Rail Track Hard to round 60

Unlocked at rank 40. Objective: 2000 cash to start, but no cash from popping bloons or at the end of a round.

Attrition
Difficulty: Bloonraker Hard to round 60

Unlocked at Rank 70. Objective: No selling towers and every 10 rounds, your towers downgrade one level. If a tower has no upgrades, it is destroyed.

Self-Destruct
Difficulty: Monkeys vs Bloons Hard

Unlocked at Rank 80. Objective: Can't sell towers and every 5 rounds your most expensive tower self-destructs. Farms are disabled.

Trivia

 * Road items (Road Spikes, Monkey Glue, and Pineapples) are available on any challenge, including those that have limited towers.
 * These are similar to the Daily Challenges and Special Missions in BTD5.
 * In the Lollipop challenge you can still place a Dart Monkey by using the hotkey D.
 * In the Self Destruct challenge, if you have no towers on the fifth round, the game crashes.