Nagaropamasutta
“Yato kho, bhikkhave, rañño paccantimaṁ nagaraṁ sattahi nagaraparikkhārehi suparikkhataṁ hoti, catunnañca āhārānaṁ nikāmalābhī hoti akicchalābhī akasiralābhī. Idaṁ vuccati, bhikkhave, rañño paccantimaṁ nagaraṁ akaraṇīyaṁ bāhirehi paccatthikehi paccāmittehi.
The Simile of the Citadel
“Mendicants, when a king’s frontier citadel is well provided with seven essentials and gets four kinds of sustenance when needed, without trouble or difficulty, it is then called a king’s frontier citadel that cannot be overrun by external foes and enemies.