Perma-Spike (sometimes referred to as Permaspike) is the final upgrade of Path 3 for the Spike Factory in BTD6. Although it produces spike piles at a significantly slower rate (from every 1.75s to every 4.85s, excluding Path 2 crosspaths), it produces much more powerful spike piles that pop up to 50 bloons per pile (80 bloons with Bigger Stacks) and deal 10 layers of damage per spike for a total of 500 damage per spike pile (800 with Bigger Stacks), making it extremely powerful for stockpiling spikes. Despite the name, but in accordance with the upgrade's description, the spikes are not actually permanent. The spikes last between rounds and have a longevity of 4 rounds each or 300 seconds, whichever comes first.
The above page contains in-depth information on how to use Perma-Spike, and also includes a very useful list of towers that synergize with Perma-Spike.
In short, Perma-Spike is exceptionally powerful when very careful spending and stalling techniques are implemented, due to the nature of this tower. It relies heavily on stockpiling its spikes effectively through various means of stalling.
Overclock works exceptionally well with Perma-Spike and even more so when paired with Stronger Stimulant. In fact, it is still slow enough not to consume Alchemist buffs faster than can be re-supplied.
Note that leaving the game and re-entering will no longer erase the spike piles as of Version 12.0. Interestingly, Long Life Spikes and Deadly Spikes spike piles can be erased.
Be cautious of using Snowstorm, as Perma-Spike without White Hot Spikes (or using an MIB) cannot affect frozen MOABs, allowing them to cross Perma-Spike undamaged, especially if using 'smart' targeting, where all spikes end up in one pile. Note that Acidic Mixture Dipcannot grant frozen-popping, despite granting lead-popping.
Due to the nature that occasional server reconnections may be caused by the extreme lag on the device, even if the spike piles can be saved, it is better off not using Perma-Spike for Co-Op Mode.
Likewise, relying on Perma-Spike to beat rounds with D.D.T.s are pretty much have to be done within just one run due to Acidic Mixture Dip not saving upon map reloads. Thus, if the player loses all spikes on rounds like 90, 93, 95, and 99, they're likely doomed to lose as reloading for another try will result in the D.D.T.s to eat all of the spikes that was placed earlier and end the game.
Overall, Perma-Spike has been nerfed heavily overall since it was first released. Initially, it was considered one of the most overpowered upgrades in the game due to the 0-2-5 Perma-Spike's capability to solo until around the early 90's when given sufficient lead-popping support, all for a very low cost. Its power was reduced drastically in Version 2.0, but remained a useful upgrade for tanking various types of bloons in the late-game. Throughout later updates, it got a few nerfs but gained a few quality-of-life buffs for both its weaker 2-0-5 crosspath and for Perma-Spike itself.
Perma-Spike price increased ($22,000 → $25,000).
Perma-Spike now attacks twice as slow (3.0s → 6.0s).
Permaspike with Bigger Stacks now has larger stacks (60 → 70; effectively now up to 700 damage per spike pile for Permaspike with Bigger Stacks)
Perma-Spike duration decreased (350s → 300s).
Perma-Spike price increased ($25,000 → $30,000).
Perma Spike max rounds reduced (4 → 3).
Perma-Spike spike piles can now be saved (i.e. on any type of game save, including Co-Op saves)
Perma-Spike now attacks 4x as fast for the first 2.5s of a round.
Perma-Spike lifespan increased (3 → 4) by the Deadly Spikes round duration buff
Description changed from "Spikes become almost permanent." to "High damage spikes become almost permanent."
Contrary to popular belief, the Perma-Spike does not have a cap on the number of spikes, or a spike pile limit. The only limit is from the spikes decaying.
Between Version 2.0 and 11.2, a 0-2-5 can achieve a spike pile that can store around 65,000 pops worth of spikes if allowed to create a full stack.
The description of the upgrade reads "High damage spikes become almost permanent.". However, the spikes from Perma-Spike only last 3 rounds or 5 minutes, whichever is sooner.
This makes sense, though, because almost permanent means that the spikes will disappear at some point.
Contrary to popular belief, x-0-5 Permaspike did not have the same attack speed as x-0-4 Deadly Spikes in v1.0 at the game's launch. The attack cooldown actually dropped from 2.2 seconds to 3.0 seconds, later to be nerfed to the current value of 6.0 seconds.
Interestingly enough, before Overclock became nerfed in Version 14.0, activating the Overclock ability onto a Perma-Spike would cause the Perma-Spike to attack at the same rate as before the pre-2.0 nerf, as the former +100% attack speed bonus balances out the halved attack speed nerf to Perma-Spike.
Although Permaspike was not specifically stated to be buffed, it got indirectly buffed by the Deadly Spikes round lifespan buff by +1 round.
Prior to the Overclock reworks, a 0-2-5 Perma-Spike where the 0-2-3 Spike Factory portion was discounted by two Monkey Commerce villages would allow Overclock to last 45 seconds instead of 30 seconds. This strategy was used popularly by other players on CHIMPS, but this no longer works as of Version 21.0 and later because of the reworks.
The Version 23.0 Spike Factory attack speed overhaul has improved Path 1 crosspathing while retaining viability for 0-2-5. So overall, this is a buff to the opposite crosspath. Similar occurs for Super Mines.
Ninja Kiwi addressed that Permaspike was bugged to not inherit the base Spike Factory speed buff, so it doesn't actually attack every 4.77s without Path 2 crosspaths. In that case, it nerfed the 0-2-5 unexpectedly. This was fixed on Version 23.2, but to 4.85s instead of what calculationally should be 4.77s, although this was likely to balance out the 0-2-5 attack speed back to what it was formerly.
Calculationally, 0-1-5 should have received an attack cooldown of 3.82s, and for 0-2-5 it should have been 2.67s.
So technically speaking, the 0-2-5 crosspath is still overall nerfed, albeit just by 0.02s, which generally speaking makes little difference to its overall stockpiling power.