System Integrity Protection (SIP) provides a lot of security benefits and I wouldn't recommended turning it off like the above post suggests. Luckily, there's another way to make Hammerwatch work.
1. Find your Hammerwatch app bundle (likely in your Applications folder on your main hard drive).
(Side note: does the Steam version of the game run as expected? I know Steam defaults to storing games in Hard Drive/Users/user name/Library/Application Support/Steam/steamapps and sometimes not as application bundles at all.)
2. Right-click (or control-click) the bundle and choose "Show Package Contents"
3. Navigate to Contents/Resources/game/Hammerwatch.app and right-click to show package contents again.
4. Navigate to Contents/MacOS/osx
5. In the menu bar choose "File > New Finder Window", then choose "Go > Go to Folder..." and type /usr/local/lib.
6. Choose "File > New Folder" and name it lib. You'll have to enter your administrator password.
7. Copy the contents of osx to lib.
I think Hammerwatch is designed to copy these files automatically, but SIP prevents it because it's Apple's new security policy that software cannot write to certain locations in the file system, including where I think Hammerwatch wants to (/usr/lib). As it turns out, Apple allows writing within /usr/local, which can act as an alternate place for files that are normally expected to be in /usr.
It would be nice if Hammerwatch didn't have to store essential files in such an obscure location, for all sorts of reasons (ease of complete uninstallation, for instance). I admit, though, that I'm not a developer and don't have to solve these programming challenges.