At the end of Gotham: Season 4, with several giant game-changing explosions, the bridges connecting the city to the outside world fell into the river, isolating everyone left behind and sending the streets spiraling into chaos.
Ra’s al Ghul and “Proto-Joker Number Two” Jeremiah Valeska’s plot to level Gotham had come to pass and now it was up to Jim Gordon and Bruce Wayne, along with few loyal allies, to protect the remaining citizens from the villains who were about to run roughshod over the entire city.
Welcome to No Man’s Land. Which wasn’t just the title for Gotham’s Season 4 finale, and now the subtitle for Season 5’s entire run, but also the name of a famous Batman comic book crossover arc from 1999 created by Jordan B. Gorfinkel. Like most comic book references on Gotham though (like the Court of Owls and Killing Joke), this one will be done with a heaping helping of twists and tweaks, given that Batman himself isn’t a character on the show.
Source: IGN.com Gotham: No Man's Land Explained