Co-op Mode

 is a new mode added to Bloons TD 5 in an update that was released on 26th March, 2013. It is unlocked at Rank 10. It involves two players cooperating on a randomly chosen game of BTD5 on a randomly chosen map and difficulty, based on the players' choices.

Reverse Mode on tracks is random, unless it is specified for a Private Match. Private Matches can have difficulty, track and direction specified by the player who created the private match. Players are limited to placing, controlling, upgrading and selling towers on their half of the map. This can also provide some advantages, such as Dartling Gun control and banana collecting able to be separately performed by two players simultaneously. Co-op Mode is now available on iOS and Android.

Cooperation
Players can communicate via chat, which is accessible by pressing the button. Players can also choose to request cash from their partner, sending them the alert MSG: Your ally has requested some cash. Their partner can choose to donate cash to their partner in increments of $0.5 whenever they like, up to $500 or their current amount of cash (whichever is less) every time they donate. Donations can be made to and from both players and are unrestricted in when they can be made and how much total cash can be donated. If a player accumulates more than $5000, they will send $5000 instead of $500.

Griefing
Griefing is a method of angering people while playing games. Here's a list of types of griefing.

Maps
There is both a new map and an old map for each difficulty:
 * Beginner: Z Factor, Alpine Lake
 * Intermediate: Snake River, Haunted Swamp, Jungle, Country Road
 * Advanced: The Eye, Cash Money, The Great Divide
 * Expert: Castle, Spider Map

Bugs

 * The screen will sometimes black out for a second, then the current round will finish, but it is impossible to go on to the next round.
 * On rare occasions, towers will automatically upgrade, and some even into impossible upgrade combinations. An example is a (3,4) Dart Monkey.
 * Sometimes the game freezes and although you can still chat, the game won't play. This doesn't happen if the game is lagging.
 * Quite commonly, the round will finish when a low-ranked bloon is still on the track. It will still be able to be popped.

Trivia

 * Players who own Premium Upgrades will have them activated, but the other player will not be able to gain access to them, with a few exceptions such as Triple Lives.
 * When a game finishes and the player wonders whether to continue, Freeplay Mode can only be played if both click on yes, if one does not want to participate, the game ends.
 * If the ally disconnects from the game there will be an option to play solo, which is arguably easier than playing Co-op. The player will not get the Co-op medals for completing the track, although the player will receive the full amount of Monkey Money if the medals for that track and difficulty have not yet been earned. If the track is incomplete in regular single-player mode, the player will earn the medal for single-player mode instead.
 * The Monkeys playing Co-op mode appear to have been playing for a long time without sleep, as they have wrinkles under their eyes.