阅读下面短文，在空白处填入1个适当的单词或括号内单词的正确形式。

The Xi'an City Wall is the most complete city wall that has survived China's long history. It _______ 41  (build) originally to protect the city______ 42   the Tang dynasty and has now been completely restored (修复). It is possible    43_______  (walk) or bike the entire 14 kilometers.We accessed the wall through the South Gate. The wall is 12 meters high and from here you can see streams of people moving inside and outside the City Wall.After   44_______   (spend) some time looking at all the defensive equipment at the wall, we decided it was time for some action and what 45_____ (good) than to ride on a piece of history!We   46______   (hire) our bikes from the rental place at the South Gate. My bike was old and shaky   47_____   did the job. It took us about 3 hours to go all   48_______   way around the Xi'an City Wall. Supposedly you can do it in two hours, but we stopped at the different gates and   49________   (watchtower) to fake pictures or just to watch the local people going about their   50_______   (day) routines.