Mars Defense Perimeter platforms were large unmanned defense platforms. The Mars Defense Perimeter in the Sol system included hundreds of these platforms just beyond the orbit of Mars and near the system's asteroid belt. (The Dominion War Sourcebook: The Fires of Armageddon)
The platforms were unable to hold off the Borg attacks in 2367 and 2373. Following the second attack they were upgraded, though it was still not enough to stop the massive Breen attack in 2375. However, without the system the casualties on Earth would have likely been significantly higher.
The platforms were spherical in shape with a radius of 60 meters and were powerful weapons. The weapons systems on the platforms included type-XV planetary defense phaser arrays that were placed equidistantly around the sphere so that any given target can be targeted by at least two phasers. Six torpedo launchers capable of launching photon and quantum torpedoes, are also placed equidistantly around the platform.
Powerful regenerative shields protected the platforms and were equipped with an auto destruct system that created a massive explosion if the platforms were approached too closely without proper authorisation. Normally stationary they were capable of moving at impulse speeds.
The platforms also included a self-replication system that allowed them to replace destroyed platforms.