BOMB

Summary

• 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. 
• Defensive Strategy
• A bomb will destroy an equivalent-level wall breaker at all levels. Since wall breaker A.I. targets wall junctions, place bombs at outside perimeter intersections and corners for maximum effect.
• One defensive strategy commonly used for the placement of Bombs is to position them along the outside wall, behind external buildings. The external buildings serve to delay the enemy troops until they've grouped up, so that when they attack the wall, the bomb hits many troops at once. This is most effective against low-health troops, though grouping enough bombs together can be a serious hazard for Giants, too.
• Alternatively, some players decide to place a group of bombs around a Defensive building on the outside of their base, to act as a trap for enemy Giants. Placing a Spring Trap at the entrance to these trapped defense structures makes them even more effective.
• Be cautious when upgrading a Bomb, as this will reveal its location. Combat this by relocating it during the upgrade period, since it won't trigger during that period anyway. This is useful against players who scout out your base ahead of time for revenge attacks.
• Exploiting the way the A.I. targets buildings can dramatically increase the defensive effectiveness of your bombs. For example, a group of 2-3 bombs placed between a pair of resource collectors is very likely to destroy an entire group of Goblins as they pass from one collector to the next.