Extra deadly spikes can pop through 2 layers per spike and last another round.
~ BTD6 description as of Version 20.0
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. Additionally, all its spikes last for 3 rounds instead of 2. Like Long Life Spikes, spikes dispensed from Deadly Spikes last for 140 seconds.
It costs $2,975 on Easy, $3,500 on Medium, $3,780 on Hard, and $4,200 on Impoppable.
Deadly Spikes is a somewhat situational upgrade whose benefits rely primarily from long-term stalling, and it is prominent compared with Long Life Spikes. The upgrade alone is more expensive than adding an external buff, but there can be a few select occasions where just adding this upgrade can be useful. For example, it can tank leaking Camo Bloons reasonably well, but it will find difficulty beating the Camo Ceramics on Round 51 or the rushes of Camo Rainbows on Round 56 if not enough stalling is done. But, if stalling is done properly, it can easily tank a good chunk of camo bloons if granted external support.
In most situations, it's better off buying Deadly Spikes only when 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 and 56. 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. The advantage with Deadly Spikes though is its stockpiling potential, allowing its overall damage to become far better than Spiked Balls if used wisely.
Again, like Long Life Spikes, it's better off going for a 0-2-4 Deadly Spikes instead of a 2-0-4 Deadly Spikes, because the spikes refresh faster and a 0-2-4 Deadly Spikes synergizes better with Alch buffs.