The Harmony Towers are ancient arcane constructs. There were designed to be used in tandem as a simultaneous arcano-technical offense and defense. The Zebra federation surrendered under threat of being the tower system's target. The system was fired successfully only twice.
Joy Prime is one of the two remaining functional towers at the onset of the Fourth Era. It contains the bearer of the Element of Laughter, Pinkie Pie. It powers Baltimare and (via the Sympathy Circuit) Las Pegasus.
Charity Prime is intact, but was never brought fully online and sits non-operational. It was designed for the bearer of the Element of Generosity, Rarity.
Faith is a satellite orbiting the planet, and the adapted stasis tower of Rainbow Dash. It held the encrypted Harmony Circuit activation keys and was designed to sync each tower's control signals. It was disabled by Bon-Bon and the Bureau of Whispers during the Second Fall, but remained in orbit. Prince Solstice was able to gain access via Amity II, and used Faith as his orbiting throne room.