Survival strategy romp They Are Billions has left Early Access, unleashing the zombie horde and giving players a meaty 48-mission campaign to fight through. Check out the New Empire campaign trailer above.

They Are Billions splices city building with tower defence in an RTS besieged by the undead. You'll need to collect and manage precious resources, plonk down buildings, recruit soldiers and build fortifications, all so you can survive the next wave of hungry cadavers. And while there are incalculable monsters waiting outside the city, it only takes one of them to start an infection that can rapidly transform your sanctuary into a lively graveyard.

With the introduction of the campaign, there are now two ways to play. The survival mode gives you a random map and a time you have to survive for, while the undead smash down your walls. The campaign mode, meanwhile, tasks you with expanding the New Empire by building colonies all over the world and protecting them with the Imperial Army. Across the 48 missions, you'll get 90 techs that will you evolve your colonies, and you'll be able to learn about the origins of the infection.

I've not played since the first days of Early Access, where I thought it was a promising, stressful RTS. It can be exhausting, trying to keep back the tide of corpses, especially when you have to be so diligent while only a few zombies need to slip past your defences to cause chaos. There are always fires to put out and it's constantly escalating. I kept coming back after each failure, though, updating my fortification designs and thinking up new ideas to hold back the tide. They're great zombies, too. Each of them is an individual, but when they spot a snack or hear something interesting, they'll quickly start to gather, eventually becoming a rotting mass that takes up most of the screen. The game can support up to 20,000 units.

They Are Billions is out now on Steam and the Humble Store for £21.59/$26.99.