Bombs are inexpensive traps that remain hidden until they are activated by an attacking troop moving into their trigger radius.
Once triggered, Bombs must be re-armed, for a small fee, in order to trigger again during an enemy raid. For example, if one’s village successfully defends against an attack, but several Bombs are triggered, then any attacks after that will not have to deal with those Bombs until they are re-armed.
Although they are the weakest of all the Traps, Bombs can still have a positive impact on the effectiveness of a village’s defense if properly placed. But only if they are always re-armed after triggering.
Even though Bombs are classified as “Defensive Buildings”, they are not buildings and do not need to be triggered/destroyed for your enemy to reach 100% destruction (nor do they count toward it).
The Bomb explodes approximately 1.5 seconds after an enemy comes within 1 tile of it, causing splash damage to every enemy ground troop within 3 tiles. This delay means that you’ll want to place the bomb in an area where troops are likely to remain for a period of time, like right next to a building they’re attacking.
By itself, a single Bomb will not do a great deal of damage to even the weakest of Troops (although every bit helps). However, several Bombs in a cluster can significantly damage or even destroy an entire group of Archers or Goblins.