@sanderd17: By default, the Slackware package manager doesn't use dependency tracking. I typically install everything from the single installation DVD.
After that, an easy way to install more software is to use scripts provided by a third-party site called SlackBuilds.org. A program that make SlackBuilds even easier to use is sbopkg.
0 A.D. 0.0.20 is presently in the SlackBuilds repo: https://slackbuilds.org/repository/14.2/games/0ad/
Installing manually is an option some Slackers choose though. Just thought I'd pass along the info for future reference.
And actually I see some of that information is mentioned on the 0 A.D. site under Download -> Linux section ( https://play0ad.com/download/linux/#Slackware )
That page may need to be updated slightly, as Slackware 14.2 was released on July 1, 2016.