Matsya Purana — Maya’s Nectar-Reservoir in Tripura and the Revival of the Slain in the Tripur...
दुर्गं वै त्रिपुरस्यास्य न समं विद्यते पुरम् तस्याप्येषो ऽनयः प्राप्तो न दुर्गं कारणं क्वचित् //
durgaṃ vai tripurasyāsya na samaṃ vidyate puram tasyāpyeṣo 'nayaḥ prāpto na durgaṃ kāraṇaṃ kvacit //
Truly, for this Tripura there was no city equal to its fortress; yet even it met with this calamity. A fortification is never, in any case, the real cause of safety.
It does not describe cosmic Pralaya directly; it teaches a parallel principle of impermanence—worldly structures, however mighty, are not ultimate safeguards against downfall.
It cautions rulers and householders not to rely only on external defenses (wealth, walls, power). True security is grounded in dharma, right conduct, and prudent governance rather than mere fortification.
Architecturally it downplays the absolute protective value of even the best durga (fort). As a Vastu-adjacent takeaway: design and fortification help, but ethical order and wise policy are presented as the decisive “cause” of stability.