Seal toward your strength
The invasion has landed inside your framework. What now? Direction decides the fight: seal the side that points away from your thickness, so the invader's only road leads back toward your wall — where every step it takes gets weaker. Sealing the wrong side pushes it out into open board, exactly where it wants to be. Your wall here runs up the left and along the bottom: cap the invader from above and it must crawl toward the wall.