Spikes last much longer and can carry over one round.
~ Upgrade description in BTD6.
Long Life Spikes is the 3rd upgrade of Path 3 for the Spike Factory in BTD6. The spikes produced by the Spike Factory can pop the same amount of bloons as the base form (which is 5, or 10 with Bigger Stacks, plus one more if the One More Spike Knowledge Reward has been applied). However, the spikes look different, now appearing as blue spikes. These spikes can last much longer than the other spikes produced by the Spike Factory. They can last up to two rounds, or 140 seconds whichever is shorter.
Always purchase this upgrade on single-entry-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.
Path 2 upgrades will add 120% extra pops with a fast refresh, while Path 1 upgrades will add only 100% extra pops with a slow refresh but allows Lead-popping. It is worth noting these two facts.
Avoid leaving the game, as the stockpiled spikes do not carry over between saves.
It's been proven that Long Life Spikes does NOT decrease spike amount when purchasing Bigger Stacks. Click for the link.
One 0-2-3 Spike Factory costs $4690 on Hard Difficulty. This is useful information when purchasing towards a Perma-Spike, as double-discounting the whole 0-2-3 Spike Factory with two Monkey Commerces will bring down the total cost of the 0-2-3 Spike Factory to ~$3752, thus bringing down the total cost of Perma-Spike to be ~$39932, barely below the threshold for Overclock's duration penalty for $40,000+ towers.
Long Life Spikes is a rather expensive but highly versatile upgrade once purchased. By allowing each spike pile to last between rounds, it is possible to stall rounds in order to stockpile many spike piles that can become carried over to the following round. Generally speaking, when attempting to pursue towards maximizing the potential of Long Life Spikes, it is worth purchasing Faster Production and Even Faster Production alongside to allow maximized net popping power stockpiles. Similar to the much more powerful Perma-Spike, Long Life Spikes is a deadly force when combined with Berserker Brew/Stronger Stimulant and Jungle Drums. Similar strategies can be used for the succeeding upgrade Deadly Spikes.