Bloon Trap is the final upgrade of Path 1 for the Monkey Engineer. It costs $2975 on Easy, $3500 on Medium, $3780 on Hard and $4200 on Impoppable. Once bought, the Engineer will automatically deploy a Bloon Trap around any part of track within the Engineer's own radius. The Bloon Trap will make bloons that come over it be destroyed entirely, though no money is awarded for such bloons (directly). It does not destroy MOAB-class bloons or camo bloons (only in BTD5 Mobile), which fly over it, unlike spikes. It can hold up to 250 bloons. However if bloons are in crowds it can be full almost immediately. When full, the trap will explode, giving the player cash, but if emptied manually, you get double cash. Upon emptying, a trap can award up to $1100. Only one trap can be active per Monkey Engineer.
In BTD Battles on iOS and Android, if you sell a Bloon Trap Tower, its trap, if still not full, will still be there and still trap bloons! The bloon trap appearance, however, still applies to other games too.
On older versions of BMC Mobile in the SD version, Bloon Trap will glitch out and give $500-$725 cash every few times you tap it. This has been fixed in a later update.
Also, when a DDT runs over a Bloon Trap, the trap will give $11 or $14 cash.
Although traps always explode eventually, Monkey City claims that every trap is sent to the Bloon Containment Unit to safely dispose of all the trapped bloons. Unless the explosions are really animations of traps warping to the Unit offscreen, this is a self-contradictory scenario.
The Bloon Traps yield less money as the rounds go higher in the online versions, however, Bloon Traps always yield the same amount in the mobile version. Thus it is more worth it to buy Bloon Traps in the mobile versions than it is in the online versions.
In Bloons Monkey City Flash, it starts to dramatically lose value after Round 15, where the value of Bloon Traps reduces to 65% of its original value. After Round 22, it drops again to 20% of the original value. After Round 27, it drops to a tiny 5% of the original value.
In BTD Battles mobile, Bloon Traps engineers are less effective against Grouped Normal Rainbows than Rainbow Regen Rushes for the exact same price. This is because the Cleansing Foam can de-regen the regen bloon property while the Bloon Trap can trap the de-regened Rainbows. Grouped Normal Rainbows, however, are completely unaffected by Cleansing Foam, while Bloon Traps cannot easily trap more than 10 Rainbows per trap.
In Flash, Bloon Traps cannot be used for regen farming. After filling up Bloon traps from the same engineer twice, the next bloon traps yield no money whatsoever.