Achilles' Bloon is a blimp that takes 220 hits to destroy, but that cannot be damaged in most regular ways. It is totally immune to any damage caused by towers, special agents, or most items. However, it has an extreme weakness (its Achilles' heel). For unknown reasons, road spikes are the only things able to damage it - it takes 20 sets of road spikes to destroy it (spikes made by Spike Factories do not count). However, it will not show any damage until it suddenly pops (so you won't find out that this is its weakness unless you actually put 20 sets of road spikes out). When it pops, it does not release any other bloons. This bloon appears on Round 62.