The file (starts at address $0800) is segmented in the following parts:

$0800 - $0bff   Split table
$0c00 - $0cff   Some kind of "zeropage", for saving single indipendent data
$0d00 - $14ff   Charset 0
$1500 - $1cff   Block definitions of charset 0
$1d00 - $24ff   Charset 1
$2500 - $2cff   Block definitions of charset 1
$2d00 - $34ff   Charset 2
$3500 - $3cff   Block definitions of charset 2
$3d00 - end     Levelmap


The categories themselves look like this 
(the following hexdumps depend on the level example):



Split table
============

$0800 - $0803:    Table entry on position 0
$0804 - $0807:    Table entry on position 1
$0808 - $080b:    Table entry on position 2
...
$0bfc - $0bff:    Table entry on position 255


Example

Pos. 0:
$0800: 00 00    "line" (low/highbyte)
$0802: 03       "what": 00=BG, 01=MC1, 02=MC2, 03=CHR
$0803: 02       "value"

Pos. 1:
$0804: 00 00    "line" (low/highbyte)
$0806: 00       "what": 00=BG, 01=MC1, 02=MC2, 03=CHR
$0807: 09       "value"

...

Pos. 255:
$0bfc: ff ff    "line" (low/highbyte)
$0bfe: ff       "what": 00=BG, 01=MC1, 02=MC2, 03=CHR
$0bff: ff       "value"




Zeropage
=============

This zeropage contains more values than listed here,
but not all of them belong to the level. Some of them 
are setting of the editor (eg. $0c0c: number of the 
currently used tile). The interesting values are:

$0c00: 14 00    Level size - columns (low/highbyte)
$0c02: 5c 01    Level size - rows (low/highbyte)
$0c04: 0d       Initial color M1
$0c05: 00       Initial color M2
$0c06: 00 00    Map editor view: 
				coordinate of upper left tile (row)
$0c08: 50 01    Map editor view: 
				coordinate of upper left tile (column)
$0c0c: 6f       Map editor view: current tile
$0c0d: 00       Initial color BO ($d020)
$0c0e: 0d       Initial color BG ($d021)
$0c26: 02       Initial charset
$0c28: 09       Block editor view: fixed color 0
$0c29: 08       Block editor view: fixed color 1
$0c2a: 0e       Block editor view: fixed color 2




Block definitions
====================

Addresses depending charset 0:
$1500 - $15ff:  Upper left tile character
$1600 - $16ff:  Lower left tile character
$1700 - $17ff:  Upper right tile character
$1800 - $18ff:  Lower right tile character
$1900 - $19ff:  Color of upper left tile character
$1a00 - $1aff:  Color of lower left tile character
$1b00 - $1bff:  Color of upper right tile character
$1c00 - $1cff:  Color of lower right tile character

depending charset 1 the tables are located at $2500-$2cff
depending charset 2 the tables are located at $3500-$3cff.




Levelmap:
============
The tiles are stored rowvise in the memory. In the level example the
length of the rows is 348 ($015c)
	
$3d00: 6f       Tile row 0, column 0
$3d01: 6f       Tile row 1, column 0
$3d02: 6f       Tile row 2, column 0
...
$3e5b: 03       Tile row 347, column 0
$3e5c: 6e       Tile row 0, column 1
$3e5d: 6e       Tile row 1, column 1
...
$3fb7: e4       Tile row 347, column 1
$3fb8: 78       Tile row 0, column 2
$3fb9: 6e       Tile row 1, column 2
...
$4113: 00       Tile row 347, column 2
...
...
$582f: 01       Tile row 347, column 19




Original document by Rapid Eraser/Forum64.de
Translation by Veto/Oxyron
Corrections and proposals are welcome :)