Ba Sing Se means "the impenetrable city". This is because of it's walls. There is an outer wall, and an inner wall. The only way to gain entry into the city was via and Earhtbender(s) lowering or opening part of the wall.
The outer wall was only ever penetrated properly by Uncle Iroh, in which he and his soldiers waged war on Ba Sing Se for 600 days. It was more recently penetrated by [[Princess Azula:char:47768]'s drill, but was defeated by The Gaang. The walls were more recently demolished by the Dai Li under the control of Azula.
Last Great Earth Kingdom City
After Omashu was captured by the Fire Nation, Ba Sing Se was the last great city in the Earth Kingdom. Sadly, recently it too was captured by the Fire Nation, leaving no uncontrolled cities in the Earth Kingdom.
Figureheads and Authority
For the time in which the show visited Ba Sing Se, the King was just a figurehead. The actual city was controlled by the Dai Li, the police of Ba Sing Se that maintained order. The King had no knowledge of the city or the war that was raging on. If someone talks about the war inside the walls, like Jet did, are brainwashed as a way to maintain order.
Recently, after Jet visited Ba Sing Se and revealed the concepiracy to the King, Long Feng, leader of the Dai Li was arrested. He was intent forming a coup to regain order, so he handed the Dai Li over to Princess Azula, who along with Ty lee and Mai had infiltrated Ba Sing Se dressed as the Kyoshi Warriors, so that she could take the King down. But, when Long Feng was released and attempted to double cross Azula, the Dai Li didn't know who to listen to, but in turn listened to Azula, and Long Feng bowed down to her.