Kṛṣṇasya asāṃnidhya-kāraṇaṃ — Śālva–Soubha-vṛttāntaḥ
Why Kṛṣṇa was absent; the Śālva and Saubha account
प्रकृत्या विषम दुर्ग प्रकृत्या च सुरक्षितम् । प्रकृत्या चायुधोपेतं विशेषेण तदानघ,निष्पाप नरेश! द्वारका एक तो स्वभावसे ही दुर्गम्य, सुरक्षित और अस्त्र-शस्त्रोंसे सम्पन्न है, तथापि उस समय इसकी विशेष व्यवस्था कर दी गयी थी
prakṛtyā viṣamaṃ durgaṃ prakṛtyā ca surakṣitam | prakṛtyā cāyudhopetaṃ viśeṣeṇa tadānagha niṣpāpa nareśa |
Vāyu said: “O sinless king, O blameless one—Dvārikā is by its very nature difficult to approach; by its very nature it is well-protected; and by its very nature it is furnished with weapons. Even so, at that time special defensive arrangements were made.”
वायुदेव उवाच
Even when a realm is naturally strong—geographically secure, well-guarded, and well-armed—wise rulers still add special precautions when circumstances demand. Natural advantages do not replace vigilant responsibility.
Vāyu describes Dvārikā’s inherent impregnability and armament, and notes that, in the specific situation being discussed, additional defensive arrangements were implemented beyond its usual protections.