Agni Purana (Jyotisha Selections) · by Vyasa
Chapter 2 · War Astrology: Yogas, Nakshatras
युद्धजयार्णवः (योगाः नक्षत्राणि)
एवं दुर्गस्य विन्यासं बाह्ये स्थानं दिशाधिपात्।) आगन्तुको यदा योद्धा ऋक्षवान्स्यात्फलान्वितः
In the full Agni Purana this is chapter 128, verse 10.
This is how the fort is laid out. Outside is the place of the lords of the directions. When the fighter who comes from outside has the nakshatra (star) with him, he gets the fruit.
Simple meaning: The fort figure is complete. A fighter coming from outside succeeds if he has the right nakshatra.
कोटमध्ये ग्रहाः सौम्या यदा ऋक्षान्विताः पुनः ।
With good planets and nakshatras in the middle, those inside win. The one who comes from outside is beaten.
प्रवेशभे प्रवेष्टव्यं निर्गमभे च निर्गमे(च्छे)त् ।
Enter the fort in the entering nakshatra, and leave in the leaving one. Venus, Mercury and Mars matter here.
तदा भङ्गं विजानीयाज्जयमागन्तुकस्य च ।
Then defeat comes, and the outsider wins. Starting battle in the four entering nakshatras brings the fort down.