They Are Billions becomes quickly a very hard game to win with the addition of extreme conditions. An increasing number of infected already present on the map or a shorter playing time.
Protect all sides of the city
At first, you will position a soldier on each side of your city. Then you will build wooden palisades, guard towers, you will reinforce the walls by replacing them with stone walls, surrounding them with piles.
You will put a big ballista on each side, then several and you will surround these towers of walls. A good way to turn the infected into junk is to make several rows of wall.
Add combat units
Obviously, the ultimate goal is to get Titans, these are the supreme units to beat the final wave. Accompanied by Lucifers and Thanatos. Put them behind walls.
There must be a constant number of iron, wood, stone, gold, and oil. But also to have enough workers, energy. And for that, it is necessary to be always expanding, despite the risks of having a flaw in the camp.
Do not wait until you run out of colonists or have an energy deficit. We must aim for a minimum threshold like 50 workers, 50 energy. Think to build several warehouses to be able to afford the right to train Titans.
Traps to avoid
We want to constantly optimize the city, but some techniques are too resource-intensive. Like wanting to have iron barbed wire, to have a bank, an improved farm, improved mills, improved quarries, …