Valmiki Ramayana Yuddha Kanda Sarga 3 Shloka 22
Original Shlokaशैलाग्रेरचितादुर्गासापुरैर्देवपुरोपमा । वाजिवारणासम्पूर्णालङ्कापरमदुर्जया ।।6.3.22।।
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दुर्गा: inaccessible, देवपुरोपमा: like the abode of devatas, सा: that, पुरै: city, शैलाग्रे: peak of the mountain, रचिता: to prepare, वाजिवारणसम्पूर्णा: full of horses and elephants, सा: that, लङ्का: Lanka, परमदुर्जया: difficult for anyone to conquer
"It is inaccessible like the abode of devatas as it is on the peak of the mountain. It is difficult to conquer for any one as it is full of horses and elephants."