The trial server is the same game as the subscription server, although the separation will certainly protect those investing in the title. All you have to do to access the trial server is sign up on the forums, after which instructions will appear on your account page.
There's one caveat of playing Xsyon for free, however: The team plans to hit the big reset switch on the trial server every Monday. So if you're looking to preserve your hard work, you'll eventually have to subscribe.
[Thanks to UltimateCarl for the tip!]