參考下面嘅段落,然後用同段落相同嘅語言回答問題:段落:Jaisalmer 堡<unk>在JaisalmerSome of these forts have defensive fortification wall up to 20 km long, still surviving urban centers and still in use water harvesting mechanism.問題:Jaisalmer 堡<unk>最長嘅牆有幾長?
20公里長