The Zark Fortress belonged to the Zark species. Spaceman Spiff was captured and taken there once. It was filled with corridors and a large room containing a giant cooking pot with smoke coming out. He was going to be "sacrificed to appease the god they called Nollij (knowledge)."
In reality, it was Calvin's school, and he escaped by drawing his "atom blaster" (a rubber band), and running out the door. When he got home, his mom called the school and took him back, and he got extra homework as punishment.