Round 1

Round 1 is the first round of almost every Bloons TD game. It has consistently contained only Red Bloons, easily poppable by unupgraded Dart Monkeys. The amounts of Red Bloons vary by installment; initially 12 were spawned, but Bloons Tower Defense 3 increased it to 14 and Bloons TD 5 increased it again to 20.

In Bloons TD 6, the round cannot be naturally encountered on Hard Difficulty and its directly related game modes, as those modes start the player off on Round 3 or later. In Bloons Adventure Time TD, 25 Red Bloons spawn.

Summary
Simply add any bloon-popping (read: attacking) tower onto the map with the bloon track in range. One unupgraded Dart Monkey on a decent placement spot is usually enough to survive Round 1. There is no point tanking this round, because the red bloons give cash when popped, which will be helpful for later rounds that require much more defense; this is a tower defense game, and the aim of the game is to prevent enemy bloons from making to the end of the map. They will continue to move closer and closer to the exit if left standing, so it is best to remove all of the bloons by popping them.

For advanced players, one may choose not to pop the bloons on this round; it's more risky than just adding a single bloon-popping tower, but it can sometimes pay off compared to losing a large amount of lives per round. In BTD6, tanking Round 1 doesn't need to happen if the player also has special Monkey Knowledge (MK), as it is possible to just use the MK to neutralize this round completely. The "Bonus Monkey!" MK offers a free Dart Monkey, which solos this round, and either Mana Shield or Pre-Game Prep can just tank this round straightup.

Tips

 * Using a Dart Monkey to beat this round is the most recommended option in any game. It is the cheapest possible tower option that also can solo this round.