Bloons Wiki
Bloons Wiki
For Bloons TD 5 buildings, see Specialty Buildings.
You will not easily deal with MOAB-class without some serious power. Tier 4 Monkey upgrades require Special Buildings to unlock. Build a Special Building for an upgrade, then research that in the upgrades menu.
~ "Real Power" goal description

Special Buildings in BMC are buildings that unlock research for special upgrades. These usually involve unlocking an upgrade for a fourth tier upgrade or a third tier Super Monkey upgrade.