Grab the demo right through Steam! If you already have Steam running on your computer, just search for the Alarameth TD Demo from within the Steam client. Otherwise, you can visit the demo page on Steam here:
Direct Link: Alarameth TD Demo on Steam
This is an older version of the demo (v1.0.5), but can be manually downloaded and installed if you have no other options. Note that it lacks a number of improvements and bug fixes.
Download the demo with the links below:
Installation is pretty straightforward on all platforms. Full installation instructions can be found at the bottom of this page if you'd like a hand.
Details: Downloadable Demo
The advantages to the downloadable demo are as follows:
Instructions (Mac OS X):
Alarameth TD has been tested in a few of the most popular distros and desktop environments.
"Easy" DEB and RPM package installers are available, as is a simple gzipped folder of the game. Use whatever method you are most comfortable with.