Ring of Fire is the final upgrade for the Tack Shooter in Bloons TD 4. It is a level 4 upgrade unlocked at Rank 22. The Ring of Fire is effective against all bloons, but it cannot detect Camo Bloons without other special towers around. Unlike firing in 8 or 16 directions like its previous upgrades, it shoots fire in a solid ring surrounding the whole tower, popping bloons that touch it or come very close to it. It pops 1 layer of bloons. This upgrade costs $2125 on Easy, $2500 on Medium, $2700 on Hard and $3000 on Impoppable.
This tower usually has a high pop count after round 60. In BTD4, this tower is a lot more effective than a Blade Shooter. This upgrade is good for thinning out several Bloons packed together.
Ring of Fire in Action in BTD4
Ring of Fire in BTD4 iPhone/iPad
Ring of Fire in BTD4 iPhone/iPad shooting bursts of fire
The Ring of Fire fires at a rate slightly faster than the Tack Shooter, and there are no holes in its range (sometimes, some of the bloons can slip between the tacks). If you pack many Rings of Fire together, even Ceramic Bloons can be popped in a few seconds, although you will need 2 at a minimum. Although not the most effective against M.O.A.Bs, they still do deal some damage to them. They usually do not perform too well when facing packed M.O.A.Bs.
The Ring of Fire is also in Bloons TD 5. It is the last upgrade for path 1. It shoots out a circle of fire that can pop Lead Bloons and Frozen Bloons, pop 1 layer of bloons and can pop up to 60 Bloons per ring. The Ring of Fire also increases the Tack Shooter's range. However, it cannot detect or target Camo Bloons unless there is a Radar Scanner or a Meerkat Spy. It is almost the same as it was in Bloons Tower Defense 4.
The Ring of Fire returns in BTD6, once again as an upgrade on Path 1, but it is now succeeded by a 5th-tier upgrade, the Inferno Ring.
The Ring of Fire functions nearly the same way it did in BTD4 and BTD5, but is now much more powerful. Despite still retaining its pop limit of 100 bloons per shot, it now deals 3 damage instead of 2 (when compared to the Hot Shots upgrade) or 1 (when compared to earlier games). The cost has also become more expensive as a result, costing $3,825 on Easy, $4,500 on Medium, $TBA on Hard, and $TBA on Impoppable.
The iPod touch/iPhone version of the Ring of Fire has a greatly increased firing rate but does not shoot in a ring, rather with bursts of fire in 8 different directions. However, the disadvantage to this are the "holes" between the fire bursts. The advantage is that it can do extra damage to M.O.A.B. Class Bloons this way.
In the recent update the Ring of Fire is now a ring instead of bursts.
Lag can reduce the Ring of Fire's firing rate dramatically, especially when comparing it with the speed of the bloons.
The Ring of Fire has a slightly greater range than the Blade Shooter in BTD4.
This upgrade may sometimes be a downgrade in BTD5, mainly because it deals less damage to MOAB-Class Bloons, and it's weak against Ceramic Bloons, because the Tack Sprayer shoots out so many tacks each doing one damage.
This is the only upgrade for the Tack Shooter that allows it to pop Lead and Frozen Bloons in BTD4 and BTD5.
In BTD5, the icon of Ring of Fire is an explosion, instead of a flame, fire or similar.
In BTD5, this upgrade suffers a bug where its range can't extend past 4/1. If the player tries to use the Super Range Tacks upgrade, it won't hit bloons further than the range it usually has at 4/1, yet it will still activate.
If the player adds a Monkey Beacon, it still won't pop the bloons further than the 4/1 range. Same applies to the Pontoon Pro bonus.
In September 29, 2013, the Ring of Fire could pop 2 layers and up to 180 bloons per shot and its price was increased to $3500. However, on October 26, 2013, it was reverted back to original, but with a slightly faster speed.
This upgrade is the most useful for shutting down bloon rushes.Keeping 3 in a corner is enough to deter a densely packed group of RegrowCeramic Bloons.
It is very powerful when it is put in the center of a 3-sided curve.
Just like the Ray of Doom, when it reaches its popping power it will still go over the bloons but not pop them.
In Bloons Monkey City it is shown that the ring of fire upgrade uses thermite, an extremely hot liquid that is practically liquid metal. It would take extremely large amounts of heat and power to get the thermite as far as it does in the game.