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.
It appears to be based off of 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 Bloon and 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 BTD4 and BTD5 the Spike-O-Pult is an upgrade to the Dart Monkey. In BTD3 the Spike-O-Pult was a independent tower.
If you place a 2-0 Dart Monkey near a Dart Monkey with Super Monkey Fan Club upgrade, then if you use 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 you 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, the Spike-O-Pult upgrade shrinks the Dart Monkey operating it.
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.
The Spike-O-Pult shoots slower than the upgrade before it.
This is the first tower to become an upgrade for another tower.