The Spike-o-pult appears as its own tower in BTD3, and as an upgrade for the Dart Monkey in all versions of BTD4, all versions of BTD5, all versions of BTD Battles and all versions of Bloons Monkey City and BTD6.
In Bloons TD 3, the Spike-O-Pult appears as a tower separate from the Dart Monkey, with 12 popping power per spike ball. Its first upgrades focus on the Spike-O-Pult and the last upgrades focus on the power. Longer Range made the Spike-O-Pult increase range, while Faster Reload made the Spike-O-Pult shoot faster. Juggernaut increased the Spike-O-Pult's popping power and pierce, and Triple Shot allowed the Spike-O-Pult to shoot 3 spikes at once.
In Bloons TD 4 and Bloons TD 4 Expansion, this upgrade is the Dart Monkey's third upgrade. On Easy this upgrade costs $255, on Medium it costs $300, and on Hard it costs $325. The Triple Shot upgrade from Bloons TD 3 was renamed for Bloons TD 4. It became the Dart Monkey's 4th upgrade and was called Multi Shot. Place these towers at the end of long lines with target priority set to last for maximum effectiveness.
In Bloons TD 5, it is about 3x bigger than previous versions. The Spike-O-Pult is the third upgrade on the first path of the Dart Monkey; and it costs $425 on Easy, $500 on Medium, $540 on Hard and $600 on Impoppable.
In BTD6, the upgrade returns as the third upgrade of Path 1 for the Dart Monkey. It costs $255 on Easy, $300 on Medium, $325 on Hard, and $360 on Impoppable. It pops up to 22 (23 with Monkey Knowledge) bloons instead of 18 and deals one additional damage to ceramics. Like BTD4 and BTD5, attack speed decreases with this upgrade, specifically from per 0.95s to 1.25s.
It appears to be based on the onager, a type of catapult that uses tension to power the shot. The onager, aka. mangolnel is named after a wild species of donkey because of the intense force with which the projectile is shot, causing the device to jump forward every shot.
In Bloons TD 3, Juggernaut simply made the balls go even further. It also allowed the Spike-O-Pult to be strong enough to crack open the Ceramic layer of Ceramic Bloons, but cannot pop Lead Bloons.
The way the monkey holds the Spike-O-Pult changes as the Bloons TD series progresses. In Bloons TD 3, he holds it with his right hand, in Bloons TD 4 he holds it with his tail, and in Bloons TD 5, he holds it with both hands.
In BTD4, spiked balls won't explode if you have Splodey Darts. In BTD5 if you have Splodey Darts, it won't explode until they have popped 18 bloons or hit a lead bloon, so Splodey Darts are not very useful with Spike-O-Pult.
In BTD4 and BTD5 the Spike-O-Pult is an upgrade to the Dart Monkey, but in BTD3 the Spike-O-Pult was an independent tower. This makes it the first tower to become an upgrade for another tower.
If players place a 2-0 Dart Monkey near a Dart Monkey with Super Monkey Fan Club upgrade, then if the player uses the ability and once the 2-0 Dart Monkey has transformed into a Super Monkey, upgrade it to Spike-O-Pult then it change into a Spike-O-Pult Dart Monkey but it will shoot as fast as a Super Monkey for the duration of the Super Monkey Fan Club ability. It also happens with the Juggernaut upgrade if players upgrade to it fast enough.
If it has already been upgraded, it will turn into a Super Monkey who occasionally throws spiked balls.
In BTD4 and BTD6, the Spike-O-Pult upgrade shrinks the Dart Monkey operating it.
The Spike-O-Pult shoots slower than the upgrade before it.
This tower is most effective on First targeting when the bloons are moving away, and most effective on Last when the bloons are moving towards it.