Casualties: Unknown Multiplayer Setup for Version 1.0

There is no built-in co-op

The official Steam listing describes a single-player game. Community co-op is available through Casualties Together, a separate community modification rather than a developer feature. A recording of friends playing together does not change that product description, so treat any multiplayer guide as documentation of a mod, not of the retail game.

How the mod connects players

A setup guide says Casualties Together adds online co-op through Steam lobbies. The install tutorial has the host start the run and then wait for friends to join. One practical consequence is spelled out in that same guide: you cannot host a modded session and expect someone running an unmodified game to join. Everyone needs the same modified installation, which is also why the mod installation page matters before any of this works.

Version status for 1.0

One guide reports that Casualties Together v5.5.0 supports the full release of the game but had not been thoroughly tested at the time of writing. The project release list confirms that version number exists. Because the mod targets a moving game build, check the release notes for the version you download rather than assuming an older tutorial still matches. The mod is unofficial, so keep a backup of your saves before trying it.

What the tutorials actually show

Two video tutorials were checked. One begins by downloading the game from Steam and then separates the loader and the mod into two downloads; the other walks through a host-and-join flow with friends. Both predate or describe earlier builds, so the specific file names and menu labels may differ in 1.0. Neither is an official installation method.

Limits of this answer

No live lobby test was performed here, and no claim is made about current server availability, player counts or whether a given build still connects. If the host cannot be joined, the first things to check are that both players run the same game version and the same mod version.