You can turn Bloon Traps into a viable and very profitable weapon using a Monkey Farmer.
Build your 4/0 (or 4/1) Engineers with their range just touching a small portion of road so that you can control exactly where the trap goes.
You preferably don't want to use Larger Service Area because it will probably force your Engineer's range to touch different portions of the road
Put the Monkey Farmer in range of the portion of road the traps will be placed on
You now have a near constant defense for that strip of the road that will also give extra money
This will also give you a lot of Cleansing Foam on that spot which is a nice bonus to overall damage including the ability for Engineers to take out Camo Bloons on low levels without the need for Camo detection.
It is best advised not to use the Engineer's special building because the turrets will kill a lot of Bloons before they can even get trapped.
The Bloon Traps can be used to get extra money during Regen Farming.