sannyasyaste sukham vasi
nava-dvare pure dehi
naiva kurvan na karayan
"When the embodied living being controls his nature and mentally renounces all actions, he resides happily in the city of nine gates [the material body], neither working nor causing work to be done."
Earlier He said that the atman should feel that the organs are doing their duty and atman is not involved in that. But how to relinquish the feeling I-am-doing and leave it with the organs? For this the mind has to play a great role. He says that this body is with nine gates and so He compares it to a fort with nine gates. The nine gates are two eyes, two ears, two nostrils, one mouth and the two excretion holes. Atman is not the nose or mouth. Mind should think that all karma done is the duty of the respective organs and not of the soul. Such a thought keeps the atman happy. This way the mind also gets controlled. Mind is then steady and does not drift and wander about. He calls the holes in the body as gates in a fort. Because, it is at the gates, enemies invade and in our case the material desires are enemies knocking at the nine gates. Also, the body only has all these gates and so it is attacked and destroyed. Atman does not have these gates and is free. The body does all karma due to the various papa/ punya accumulated over infinite birth cycles.