The game itself is free, click on "Download Now" then "No thanks, just take me to the downloads"
Payment is completely optional
The download is a .quest file that needs the Quest engine to run it. You can find the download for Quest in the description of the game, or here: https://textadventures.co.uk/quest
Just note, the downloadable version of Quest only works on Windows. If you are playing on Mac or iOS, you'll need to play the online version instead, which can be found here: https://textadventures.co.uk/games/view/cmt50dll2koybs5hqny8qa/the-cave-v0-4