HATPC Reborn level files have 3 main sections - the header, the cave itself, and the list of events.
The header has 4 lines.
The first line of the header is cave title, which must be non-empty but otherwise can be anything.
The second line is the terrain type - the word "terrain" followed by a number from 1 to 5.
The third line is the background type - the word "background" followed by either 1 or 2.
The final line is the water type, which can either be one of default water types ("water clear", "water murky", or "water dark") or a custom water type defined by a water density ("sparse", "medium", or "dense") followed by an HTML color code (minus the # symbol).")
The cave itself is a rectangular matrix of ASCII symbols - several lines of equal length. Each of those ASCII symbols represents a tile in the cave. If you want the cave to be completable, it must contain at least one Hannah object (coded by a #) and at least one door (coded by a D). The rest of the tiles can be pretty much anything selected from the list of tile codes (although it's generally advisable to encase the level in terrain, coded with X).
Events are lines beginning with the $ symbol after the main body of the cave. To learn how to use events, head on over to my events guide.
In order to load your level file, it must be uploaded somewhere on the internet, such as The Interguild.
Then, you must grab the URL of your level file. If your level file is on The Interguild, the URL of the level file will be http://www.interguild.org/levels/levels/#####.txt but with the ##### replaced with the numerical level ID (found in its Interguild page URL).
Next, encode your URL with URL encoding. You can use this site to easily do so.
Finally, to play your level append your encoded level URL to http://www.droidfreak36.com/HATPC/reborn.php?cave= and navigate to that link. HATPC Reborn will automatically load your level in the newest version.