Valmiki Ramayana Yuddha Kanda Sarga 3 Shloka 20

देवदुर्गा: difficult for devatas to access, निरालम्बा: being located on peak of mountain, लङ्का: Lanka, पुनः again, भयावहा: fierce, नादेयम्: rivers, पार्वतम्: mountains, वान्यम्: forests, कृत्रिमंच: and artificial fortress, चतुर्विधम्: four kinds

"Lanka is difficult to access even for devatas located on high peaks of the mountain. It is fierce to onlookers, surrounded by four kinds of forts, like rivers, mountains, forests and artificial fortresses."