Sitting on the top of a mountain, and sporting an over-sized golden statue of Robotnik doing his pose with his arm raising forward, Dr. Robotnik's Fortress is his central base for his plots to take over Mobius and destroy Sonic in Adventures of Sonic the Hedgehog.
Damaged and destroyed on many occasions, the base has little defenses except for that of Scratch, Grounder, and Coconuts. Although it holds a massive prison block, the base is more of a home to Robotnik than an actual military base. When not having the 6-S and/or Coconuts on a mission, they are usually the ones cleaning the base. The fortress is also Sonic's usual first and last stop in preventing Robotnik's plans of taking over Mobius. It's revealed in the episode Zoobotnik, that the golden statue of Dr. Robotnik has a flippable top, so it can become a watch tower.
Robotnik's fortress is destroyed in Robo-Ninjas, and is not seen again in the next episode or in the Christmas special (in which Robotnik had a different base, located in Robotropolis), leading to the conclusion that after the base was destroyed, it was destroyed for good. Although Volume 3 of Shout Factory's AOSTH DVD Collection said that Sonically Ever After happens right after the "Quest for the Chaos Emeralds" Saga; making it up for Sonic to say "See ya soon for more Adventures of Sonic the Hedgehog". If that's the case, then Fast and Easy happens right after Robo-Ninjas while Hero of the Year happens before Sonic Christmas Blast.