You can make and share a custom level for Donner, Dasher, and .. Dynamite? by editting the level template and pressing the button. This will open a new window with a custom URL which you can use to share your level around the xmas interwebs. The rules for the level structure are specified below.
The first line contains a number between 0 and 4 (inclusive) that determines the speed (0 is the fastest). This must be followed by a comma. The next 15 lines contain 15 characters each that describe the level layout. The characters you can use in the description are:
.: empty space
$: the xmas tree
1-9,A-T: the sequence of (up to 29) reindeer
@: put this next to the last reindeer to orientate it
wxyz: the presents (can have more than one of each)
WXYZ: the doors (max one each)
Note: You must place the reindeer in successive order. For instance, to use 3 reindeer, you would place a '0' (santa), a '1', a '2', and a '3' next to each other, then add a '@' next to the 3.
Note: After '9' the next reindeer is 'A', then 'B', and so on.
Note: Each level line must contain exactly 15 characters and no spaces.