Extra deadly spikes can pop through 2 layers per spike.
Deadly Spikes is the fourth upgrade of Path 3 for the Spike Factory in BTD6. It allows all spikes of the Spike Factory to pop two layers instead of one layer, effectively doubling its power. It costs $2,975 on Easy, $3,500 on Medium, $3,780 on Hard, and $4,200 on Impoppable.
Avoid purchasing this upgrade unless Berserker Brew is not available. Berserker Brew adds +1 damage, +2 pierce, and ~+10% attack speed for the Spike Factory. Note that 3-0-0 Berserker Brew costs just $2400 on Medium; literally saving $1100 plus extra benefits, in spite of the brew effect having only 62.5% uptime.
Deadly Spikes is not reliable at soloing Camo Bloon rounds, particularly Rounds 51. It is better off choosing an alternate option to counter Camo Bloons, such as a 4-x-x Bloonjitsu or by adding in a cheap form of decamo such as Submerge and Support or Counter-Espionage.
By itself, a 0-2-4 Deadly Spikes is less reliable than a 3-2-0 Spiked Balls at general cleanup. However, when paired with Berserker Brew, Deadly Spikes becomes a somewhat decent cleanup option, though both options may still become overwhelmed by large hordes of bloons.