Because the main servers are offline, you must connect to a private server to play with others:

Avoid third-party "repack" sites claiming to offer highly compressed versions of this game. Since the official Private Server Edition is already free , there is no reason to risk your PC's security with unofficial downloads. Knockout City: Private Hosted Server Edition

from Steam or the official site .

: Many players use the XYZ Launcher (ko.city) to simplify connecting to active community-run servers. System Requirements

: The download includes a KnockoutCityServer.exe if you want to host matches for your friends.