Flanking

Some defensive structures will be triggered as soon as they’re attacked, causing them to counterattack — sometimes to devastating effect. Many of these defenses, however, can be neutralized by first flanking them before attacking them. To flank a defense, its land square must be surrounded in all 8 directions by either: your nation or one of your allies, mountains or water, a square containing a resource or Orb, or another (non-wall) defense belonging to the defending nation.

This tower is flanked

Flanking can be particularly useful against invisible defenses (see Invisibility). It’s often better to flank an invisible defense before attacking it, rather than risk triggering a powerful defense that could bring your offensive to a halt.

Next: Crumbling Defenses