Valmiki Ramayana Sundara Kanda Sarga 3 Shloka 14
Original Shlokaनेयमन्येन नगरी शक्या धर्षयितुं बलात्। रक्षिता रावणबलैरुद्यतायुधधारिभिः।।5.3.14।।
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उद्यतायुधधारिभिः ready to fight, रावणबलैः by Ravana's forces, रक्षिता protected, इयं नगरी this city, अन्येन by others, बलात् with force, धर्षयितुम् to attack, न शक्या not posible.
'It is not possible to attack this city by to outsiders as it is protected by the forces of Ravana with their weapons ready(to attack).