They Are Billions is an RTS game with an uncommon goal for most games of the same genre: survival. Set in a post-apocalyptic steampunk setting, the gameplay of TAB is a mix between city-management and world survival.
Much like with The Settlers series, you start off with a basic main structure from which you must strive to build a colony in hopes of surviving in the zombie-ridden wasteland of TAB. Requiring careful resource management, you must make tough decisions that either contribute to the growth of the colony and increase its chances of long-term survival, or make investments in short-term security measures in order to effectively respond to immediate threats.
TAB is unforgiving because the smallest oversight in your lines of defense can bring down your entire colony -- after all, these are zombies, and not only will they destroy your structures in their mindless rages, they will also infect your colonists and turn them against you. You can very easily end up with a snowball effect of decaying flesh and destruction, all because you thought one defensive tower and a single wall would be enough to hold that tiny gap.
In the spirit of TAB, we've come up with a list of other unforgiving RTS games where single mishaps can have similar cascading or outright immediate catastrophic effects for your mission and/or campaign. So without further ado, let's get on with the first of the list.