The file begins with the four-byte magic number 72371524, and then two four-byte offsets to the polygon and thing records, in that order.
Then there follow records of the following format for polygons, until the offset specified for things:
int tex_type_id ; 0 for flat, 1 for tex
int vx_order ; 0 if floor, 1 if ceil, 2 for both
; planes, 3 for clockwise vertices
char[8] tex_name ; flat or texture name, null-padded
int num_vertices ; number of vertices in polygon
num_vertices many:
float x ; Vertex co-ordinate.
float y ; "
float z ; "
float u ; Texture domain co-ordinate.
float v ; "
Finally there are the thing records, thus:
int type ; Class of thing
int angle ; Anticlockwise angle from 0-359, 0 being +ve x dir
int x ; Position
int y ; "
int z ; "