Perma-Spike

"Spikes become almost permanent."

- Upgrade description in BTD6.

Perma-Spike is the final upgrade of Path 3 for the Spike Factory in BTD6. The spikes produced by the Spike Factory pop 50 bloons per pile (70 bloons with Bigger Stacks) but deal 10 layers of damage per spike for a total of 500 damage per spike (700 with Bigger Stacks), making it extremely powerful. 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 350 seconds, whichever comes first.

Tips

 * Always purchase this upgrade on single-entry or single-exit maps near the end of the track. Then, spikes won't be used up immediately, allowing spike stockpiling to slowly make the tower more and more valuable. Even though this may make the Perma-Spike less useful evidently, it is still the best way to use it.
 * It is recommended to purchase the Path 2 Upgrades (Faster Production and Even Faster Production) over the Path 1 Upgrades (Bigger Stacks and White Hot Spikes) as Bigger Stacks only increases its pop count per spike from 500 to 700 and, although White Hot Spikes allows it to pop any D.D.Ts that may rush through a defense, a Monkey Intelligence Bureau or Acidic Mixture Dip can easily solve that problem while also providing it and other nearby towers with the bonus as well.
 * Path 2 upgrades will add 122% extra pops with a fast refresh, while Path 1 upgrades will add only 40% extra pops with a slow refresh but allows Lead-popping. It is worth noting these two facts.
 * Where Alchemists are available, it is best to purchase a 0-2-5 Permaspike nearby a 2-0-0 Alchemist at minimum, preferably a 3-0-2 (or 4-0-2) Alchemist for extra damage, pierce, and attack speed (and range, but that's irrelevant to DPS). When using an Alchemist in place of Monkey Intelligence Bureau, the Perma-Spike should be the only tower in range of the Alchemist.
 * Note that leaving the game and re-entering will erase the spike-pile. Due to the stacking nature of the tower, this will heavily reduce the power potential of it.

Strategy
Perma-Spike is one of the most cost-efficient forms of exit protection for almost all stages of the game. Despite Perma-Spike's slow attack speed, its super sharp spike piles will pack a huge punch to bloons that approach them if used correctly. At the cost of one-third of the original attack speed, each spike pile deals 10 damage with up to 50 spikes per pile and last for a significantly long time. Due to the superior long-term benefits of stockpiling spike piles, it is better off using Perma-Spike as exit protection rather than as an offensive tower.

Perma-Spike increases much of its potential via the use of stat boosts, especially those that increase attack speed. Crosspathing to 0-2-5 Perma-Spike, which increases attack speed by 122%, will effectively grant 2.2x more damage stockpiling than a non-crosspathed Perma-Spike. On the other hand, a 2-0-5 Perma-Spike grants only +20 pierce, which effectively only adds 1.6x more damage stockpiling at a slower refresh rate. Thus, it is better off crosspathing the Spike Factory with Path 2 prior to getting Perma-Spike.

In addition to internal upgrade boosts, Perma-Spike benefits greatly with external sources of stat boosts; this makes it a good combo with a 2-x-x Jungle Drums Village and a 3-0-2 Berserker Brew Alchemist (or 4-0-2 Stronger Stimulant Alchemist for slightly better results, though 3-0-2 is sufficient). Besides simple attack speed bonuses, the extra damage and pierce add significantly greater damage-stockpiling potential to Perma-Spike. In addition, the slow attack speed of Perma-Spike ensures that the Acidic Mixture Dips do not expire immediately, thus producing "permanent" lead-popping power, rendering the Path 1 Perma-Spike crosspath useless when Alchemists are enabled. However, if Alchemists aren’t available, you could always use a Monkey Intelligence Bureau to guarantee lead-popping power for it and all towers nearby.

Other methods of optimizing Perma-Spike include stalling out the rounds so that it can produce many more spikes per round in preparation for future rounds. Maim MOAB set to Last and targeting on ZOMGs will ensure that they will be stunned so that none of the towers except a single Maim MOAB can attack the ZOMG, thereby giving Perma-Spike more time to stockpile extra spikes. 0-x-2 Ice Monkeys stalling Super Ceramics are another viable method of stalling out rounds.

Optimal Perma-Spike spots are important to consider prior to placing Perma-Spike. Important aspects to consider include whether Perma-Spike is far enough from the map to provide maximum stockpiling potential, whether the spikes avoid any potential to accidentally become consumed by incoming entrance bloons, whether sufficient exit-stopping coverage is achieved, and, in the case of multiple paths, whether all paths are covered by the spikes. All of these aspects determine how "good" of a placement a Perma-Spike is. In CHIMPS Mode, it is impossible to resell to re-position the Perma-Spike placement, so be sure to plan placements prior to placement.

There are only a few maps where the Permaspike may have a few complications when it comes to optimization, which comes due to the lack of single-entry-single-exit on certain maps. Usually setting the target to Close should help target out the intersections of exits. However, there is no easy solution to optimizing Permaspike on harder maps such as #Ouch or Muddy Puddles, where there are either multiple intersection exit points or multiple disconnected exits. To solve the latter problem, Support Chinook could be used to carry Permaspike along the map each round to organize into their new appropriate exit-protection roles.

Optimal Permaspike Placements
(Construction: Will add optimal Spike Factory spots later with a gallery dedicated for Spike Factory spots. Will in addition discuss best use of Permaspike, including best upgrade combinations, best towers to pair nearby, and other good tips with Permaspike.)

Update History
2.0

Perma-Spike price increased ($22000 → $25000).

Perma-Spike fire rate decreased by 66%.

8.0

Permaspike with Bigger Stacks now has larger stacks (60 --> 70; effectively now up to 700 damage per spike pile for Permaspike with Bigger Stacks)

Trivia

 * This is one of the few towers that can destroy a B.A.D. singlehandedly if given lead popping power and sufficient time to stack its spikes. The other towers are the Legend of the Night, Ezili with her level 20 ability, the Apache Prime upgraded with the 2nd path, the Sun Temple and True Sun God if given sufficient sacrifices.
 * This has been used by many, if not, all players to beat Dark Castle C.H.I.M.P.S..
 * 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.