Brood War had exposed a flaw in the Terran defenses as to how vulnerable their workers were at whatever point in their development. Enemies relished in exploiting this weakness, dropping their troops right in the middle of groups of SCVs and sowing chaos. Terrans have since then found the solution to this problem, as Command Centers can now be upgraded to Planetary Fortresses: a Command Center with a huge double-barrelled railgun which can simply deliver enormous damage. Planetary Fortresses unfortunately cannot lift-off and cannot strike anything in high-altitude, but coupled with several Missile Turrets, the Planetary Fortress is an extremely effective defense structure, especially for small base expansions and against the new cliff-jumping Reapers enemy Terrans will be very likely to use, and the all-terrain Protoss Colossi. As a side note, Planetary Fortresses cannot change back to Command Centers.