Valmiki Ramayana Yuddha Kanda Sarga 41 Shloka 96

दुर्धर्षः one who is difficult to encounter, कपिः monkey, सुग्रीववचनात् by the command of Sugriva, चतुर्द्वाराणि the four gates, सर्वाणि all, चन्द्रमाः moon, नक्षत्राणीव stars, पर्यक्रामत moves around.

Sushena, a monkey who is difficult to encounter, went around all the four gates just like the moon goes around stars by the order of Sugriva.