新东方教材一道语法题

新东方教材上45页10题：The Forbidden City in Beijing, from which the emperors ruled by heavenly mandate, was a site which a commoner or foreigner could not enter without any permission, on pain of death.

b.which a commoner or foreigner could enter without any permission only

c.which no commoner or foreigner could enter without permission

综合比较觉得C比较合适，答案也选的这个，但是不懂最后那个on pain of death在这儿怎么理解呢，no commoner of foreigner could enter without permission, on pain of death 怎么觉得似乎少了些什么呢。

求解~~