Every race has them. It’s interesting b/c while most towers eat up a 2x2 footprint, these command more than double the building space at 3x3.
TL;DR, Is 3x3 area too much? Are they worth the extra space they take up?
Should some of them be 2x2 instead? Likewise, should there have been more of them at 3x3? it’s kind of an interesting dynamic. Like how some COs have a 100 supply cap, or have auto-gas buildings, this is sort of a penalty, but doesn’t seem to have much rhyme nor reason to them.
In general, you can usually work around this with more careful planning. Some missions like DoN have particulars where putting in an AG (anti-ground) tower has more nuiances
After the ‘.’, I point out some other notes, and honorable mentions…
==== For Terran
—Raynor - Bunkers - 100/0/0
Comes with a Shrike turret that does AG and AA. So while you don’t need infantry in them, they’re low dps on their own that it’s only good for picking off low numbers and hp of enemies. Infantry is rather mandatory for them to be effective which means you’ll need to commit supply for their intended purpose of d.
This one can upgraded with +2 slots for housing infantry (so 2 extra Marines, 1 Marauder, or 1 Firebat), and +2 armor. Lv7 ups its hp from 400 to 600 (along with the Shrike Turret).
Its closest 2x2 competitor is Mengk’s Supply Bunker which also provides not 8, but 12 supply, in addition to bunker capabilities! However, the +1 range to infantry in it is an explicit upgrade, along with the +200hp & +2 armor.
Seems like Mengsk comes out ahead here, but we’d need to take into account a whole picture, which I don’t do in this post.
—Mengsk - ESO (Earth Splitter Ordanance)
For how good it is, this seems appropriate at 3x3 footprint.
Closest 2x2 competitor is Zagara’s Bile Launcher. This one is more of a sure shot (with some leading the targets and skill), and can also hit air. However, it’s range is much less (25 vs. 75), and can only be built on creep. Also, pricey, and without Salvage
.
HH - SFP
These are “tower-ish”. They need to be used manually, but can definitely fill a defensive role. Being limited to 10, its 3x3 size shouldn’t be a big deal (although it’s still tricky to work in on small real estate like OE and DoN).
As mentioned, Mengk’s Supply Bunker got to stay at 2x2!
==== For Zerg
—Stukov - Infested Bunker
These are quite the signature tower. One thing that does give credence to its 3x3 size is when uprooted, it becomes a quadripedal monstrosity that claws at enemies at melee range, while Infested Troopers still being able to fire their guns from within. It’d look more funny coming from a 2x2 structure. Game play wise, it has Ultralisk level stats for something that doesn’t even cost gas (“Bunkerlisk” 
), albeit a whopping 4 supply and mins (so 300/0/4).
I’d say they’re appropriately sized
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Somewhat surprised that Zagara’s Bile Launcher didn’t get the 3x3 treatment, but they’re not that great, so go figure. Nor Dehaka’s Primal Wurms, nor Greater Wurms. However Glevig is a 3x3 member! But he’s summoned, is temporary, and one-of-a-kind (you can’t make more than 1 at any given time), so that puts him into a different category than the more traditional towers.
==== For Protoss
—Karax - Khahadarian Monolith (300/100/0)
These are always weird to squeeze in since all other “piece types”… pylons, Cannons, and Shield Batteries are at 2x2. Putting a few along the edge of your tower lines, or behind does take care of that. However, the squeeze gets tight where space is limited, such as OE.
I’d say they’re fine. They are powerful, like “mini-capital ships”, that can take down hybrids swiftly, while outranging nearly everything. However, making them 2x2, you’d still need to overcome its price.
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Surprised, but pleasantly so that Zeratul’s Tesseract Monolith didn’t get relegated to 3x3. Then again, his towers really break the mold (they get full level upgrades, can project, and Monoliths are just bonkers with stun, multi-target, and being permanently summoned into place).
Protoss is rather bland since vast majority of them are just “tried and true” Photon Cannons. In fact, I wrote another article thread exploring tower diversity in Coop, and Protoss did “lose that contest”…
Instead, would use other 3x3 buildings like Ebay. For Karax, I would just wall off with Pylons or Sh. Batteries. They’re 2x2, but the hp to cost ratio is much better.