Invisibility is a special ability that can be very useful for defense. It is obtained by researching the advance called Mass Illusion. Invisibility allows you to hide your defenses from attacking enemies.
For nations that have developed invisibility, their defensive structures (not including walls) are invisible to all other nations, until they are triggered. Once an invisible defense has been triggered (either by being directly attacked, or for some defenses, when the defending nation is attacked within a certain range of the defensive structure) it becomes visible for 2 minutes, and then returns to being invisible.
Invisible defenses cause invading nations to be a lot more cautious in how they attack, because they won’t be able to see what defenses you’ve employed or where you’ve placed them. Invisible defenses can be placed strategically to lure attackers into triggering one of the powerful defenses that can wipe out their progress, and even cause nearby land to become uninhabitable to them for some time.
While attacking nations can’t see invisible defenses, they do have access to some hints that can help them deduce where those unseen defenses are located. A roman numeral appears in each square of a nation’s land that is next to another nation’s land square where a defense is located. The numeral tells an attacking player how many of the adjacent enemy squares contain a defensive structure. These roman numerals (called “surround counts”) appear whether or not the nearby enemy defenses are invisible; if the defenses are invisible, they can be very useful in determining where those enemy defenses are located. If you’ve ever played the game Minesweeper, you’ll be familiar with how this works.
As an attacker, you can use these surround counts to determine where to attack next, how to flank a defense, and what squares you may wish to avoid so as not to trigger a defense. As a defender, you’ll want to position your defenses to make it difficult for attackers to determine where your most dangerous structures are located, so that they won’t be able to avoid triggering them. You can even build special defense structures, called Phantasmic Threat, that cost very little energy to maintain and have no ability to attack but serve only to be included in surround count numbers, and so help to confuse attackers.