Fortification

Building Forts along your borders will extend your influence on those borders and give you more territory.

Research the Fortification techs three times during the game, available after Military advancements at the Library. You'll reap multiple defensive benefits: garrison (and thereby heal) larger numbers of units inside, and increase the range and line of sight of Forts and Towers upon completion of research.

Forts can close the gaps in your border in vulnerable areas. They'll cost you Wealth, Metal, time, and maybe a few Citizens. Flank the gaps between mountains and forests with Forts, and you can force invaders into a crunch point. This is especially effective before air power arrives in the Industrial Age.

Repairing damaged Forts is almost always cheaper and faster than building new ones, even if you lose Citizens in the process. They're a sizable investment, especially after you build a few - the cost goes up.

Forts from around the world throughout the game: