The Kitten Kingdom is a prominent civilization featured in the Epic Battle Fantasy games. As its name suggests, it is a nation inhabited entirely by cats. Its current size is unknown, but it was once the dominant civilization on the planet due to the influence of Godcat.
Epic Battle Fantasy 2
The game begins in the Kitten Kingdom, although the scenery is drastically different from the ruins displayed in later installments; presumably, this is because it's an active settlement built and maintained with modern resources and aesthetics. Following the events of EBF1, Matt and Natalie are recovered by the residents of the Kingdom and brought back to recuperate. Upon hearing of Lance's conquest of the planet, the duo set out to stop him. For unknown reasons, this entails slaughtering a number of Kittens to "escape" the Kitten Kingdom, although it was explicitly stated through dialogue that the Kittens were friendly.
NoLegs's decision to abandon his kind in order to support Matt and Natalie prompts the denizens of the Kitten Kingdom to place a curse on him, forcing him to fight off hordes of Skeleton Cats by himself. Fortunately, the undead felines drop treasure that his friends can use to buy things from the Shop.
Epic Battle Fantasy 3
The Kitten Kingdom Ruins are a major area in the game, also providing some insight to the ancient civilization of cats. Based on the prevalence of limbless cat statues around the ruins, it likely existed for some time after Godcat took away the limbs of all cats, though it has clearly been abandoned for a while.
The empire had some of the most advanced technology ever seen by the world. Lance claims that the Protector had more powerful engines than his Valkyrie Tank, which means they had access to fission-level technology. The kittens also developed incredibly durable doors that could be operated by levers or placing special orbs on pedestals; these mechanisms have survived to the present day with minimal degradation. However, despite all this, they chose to carve their city of what appears to be sandstone. This may be cultural or just based on what materials were available in the desert.
Interestingly, NoLegs seems to understand much of the workings of the Kitten Kingdom's ancient devices and would explain it to the party if they could understand him. However, it is unlikely that modern cats possess the technological knowledge of their ancestors, given their lack of dominance in the world; their inability to rule may also be part of Godcat's actions, as suggested by EBF4.
Epic Battle Fantasy 4
This game explains the Kitten Kingdom's history and culture in much further detail, revealing that Kittens were intended to be the dominant race on Earth rather than humans, who were created to be slaves to the Kittens. Ancient humans worshipped cats as gods before an uprising of ambiguous origin drove the Kittens out of power, causing Godcat to be disappointed and curse the Kittens to lose their limbs (therefore explaining why Kittens are drawn in-game without arms nor legs). Godcat was also sealed by the crystals so that it couldn't descend from the heavens and rain fire and brimstone upon the infidels who dared challenge it. Before this happened, the Destroyer side of Godcat created Akron in order to punish the humans. Fortunately for the humans, this plan was thwarted when the party slew Akron at the end of EBF3.
Epic Battle Fantasy 5
Although the Kitten Kingdom does not make a proper appearance in EBF5, there are some hints to the existence of it or a similar civilization. Godcat remains a deific figure with her own scripture, the Book of Catus (which may be related to the eponymous Godly Book used in one of NoLegs's Cat Toys), members of the Kitten Army appear as foes (and have in fact diversified their ranks in comparison to previous games), and Telperion's summon quote makes passing reference to a lost "empire" that its feline occupants have apparently been trying to restore. In-game, however, most of the cats seem to be using trees around the Mystic Woods as makeshift homes and fortresses, most notably Laurelin.