Codes

In the Main Menu, there will be a button saying "Codes", after clicking that button a grey BG with lots of grey wires will appear, with 3 small buttons, the one in the left side says "Research Codes", the one in the right side one says "Insert Codes" (only available in-game) and the center one says "Restore Codes". Each Code has to be researched (which can be like popping x bloons and crafting y item) before being used. CAUTION:Codes can be used limited times only, after that, they have to be restored. Codes are also disabled in Special Missions. Codes are unlocked at Rank 21.

To use codes you must first research them in the "Research Codes" button, after that, you will see 3 buttons in the top, the first one says "Simple Codes" which are unlocked at Rank 21. Here is the list of Simple Codes:
 * Code: REDPOP
 * Research Mission: Pop 500 Red Bloons
 * Restore Cost: 80 MM
 * Research Cost: 300 MM
 * Effect: Deals 1 HP to every bloon on screen.
 * Code Amount: 20
 * Cooldown: 30 seconds.
 * Code: VIRAHEAL
 * Research Mission: Suffer 50 HP in total from bloons.
 * Restore Cost: 105 MM
 * Research Cost: 450 MM
 * Effect: Heals every tower on the screen for 5 HP.
 * Code Amount: 10
 * Cooldown: 1 minute

Trivia

 * Obvious reference is obvious!