Gates of Troy Digital Error - Failed to Initialise Steam

[Wadabing]

I recently purchased a digital download version of Gates of Troy off the Slitherine website. It installed without problem, but upon running the game an error message pops up with the following text:
"Failed to Initialise Steam"
Not sure where this error is coming from as the digital download page did not mention steam at all, nor is the game in Steam's library. For a test I did attempt to run the game with Steam running but no luck. Running on Windows 10, the download came with d3drm.dll in the directory folder already so it doesn't appear to be related to that.