Here's how you can download the latest official release of Tunnel.
Grab one of these downloads if you want to start making your own Tunnel cartridges. Every cartridge includes the core Tunnel files, so if you just want to play cartridges, all you need is a web browser.
Download the Tunnel engine core from the official git repository on GitLab.
The Tunnel engine core is also mirrored on itch.io by A.V. Knights.