Some advances unlock upgrades for defenses that your nation can build. For example, developing the advance Steel Construction will allow your nation to develop the advance Wall II. Having developed Wall II, you can upgrade any Wall that you build to Wall II. To do so, click (or tap) on a square containing one of your Wall defenses, and select “Upgrade to Wall II” from the menu.
If you’re using a device with a keyboard you can upgrade multiple defenses very quickly by holding down the Ctrl key while clicking on squares containing a defense to upgrade.
Next: Salvaging Defenses