Primary Monkeys Only is a game mode in BTD6. In this game mode, only Primary Monkeys and the selected Hero can be used. It is unlocked upon beating Easy Difficulty on the respective map. Easy Difficulty rules apply by default.
Tips
Be aware that you should use Dart Monkeys in order to pop the camo bloons, as none of the other primary towers can pop camos without their highly expensive 5th tier upgrades. Alternatively, use heroes that are capable of detecting camo such as level 5 Quincy or Ezili.
No primary towers can be placed onto the water unless placed in radius of an Arctic Wind, meaning this mode is harder to complete on tracks with lots of water such as Spice Islands and Peninsula. Alternatively you can spend some Monkey Money to purchase Pontoons.
Another way is to use Gwendolin as the hero, placing a Boomerang Monkey to 2-2-x and a Glue Gunner to 2-x-2. Place some other towers for towers such as x-x-2 Dart Monkeys for Camo Bloons and maybe a Bomb Shooter for the sneaky leads that get past your glue gunner. Try to upgrade your towers further, but easy doesn't last long. It'll be over before you know it! (Strategy by happybeau)
Most maps are able to be beaten to Round 100 if using Ezili as the hero. For rounds 90+, using Sacrificial Totem will grant camo detection to nearby towers, allowing D.D.T.-poppers sans camo detection (such as Bloon Crush or The Tack Zone) to become more effective. The totem is also capable of activating Bomb Blitz's life loss ability, meaning that D.D.T.s can also be instantly popped.
It is difficult to complete maps to round 100 on Primary Monkeys Only without using Ezili or Plasma Monkey Fan Clubs; other viable choices is high-level Churchill with Armor Piercing Shells ability active, and two Crossbow Masters (with Double Cross Master MK) with lots of Snowstorm abilities.