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 absorbed into the trap while no money is awarded immediately. It can hold up to 250 RBE worth of bloons. Once full, the rims of the Bloon Trap will flash blue and magenta, prompting the player to collect it for twice as much cash as the bloons would normally give. If the player does not collect the Bloon Trap after a certain while, it auto collects, giving less cash than manual collection. Only one trap can be active per Monkey Engineer. It does not destroy MOAB-class bloons or Camo bloons (except in BTD5 Mobile), which fly over it, unlike spikes.
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.
The appearance of the trap is likely a reference to the Ghostbusters' Ghost Trap.
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.
On older versions of BMC Mobile in the SD (small device) version, the Bloon Trap will glitch out and flash continuously for several seconds after multiple Bloons get captured by it, even after collected. This issue has since been fixed.
Prior to a recent update for BMC Mobile, this can trap D.D.T.s, giving 16,656 cash, or 33,312 cash if emptied manually.
This has predated the XXXL Trap's ability to trap MOAB-Class Bloons in BTD6, including D.D.T.s, other than the B.A.D.
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.