down 是介词，因为它后面接了宾语。比较：

He rolled his suitcase down. （down 是副词）

He rolled his suitcase down the hall. （down 是介词）

再比较：

He climbed down.  （down 是副词）

He climbed down the tree. （down 是介词）

类似地：

He walked along.（along 是副词）

He walked along the street.（along 是介词）

He came up.（up 是副词）

He came up the hill.（up 是介词）