The central arena is an oval 87 m ( 287 ft ) long and 55 m ( 180 ft ) wide , surrounded by a wall 5 m ( 15 ft ) high , above which rose tiers of seating .
The arena is an oval 287 ft ( 87 m ) long and 180 ft ( 55 m ) wide , surrounded by a wall 15 ft ( 5 m ) high .