About half a kilometer west of the fort is what's left of a cannon emplacement. It's got four Chinese characters carved over the door. They mean "Key to the Portals of the North," and Tamsui really was the strategic key to the whole north of Taiwan in the old days.
紅毛城西邊約半公里有一處砲台遺址，城門上劉銘傳題「北門鎖鑰」四個字，更證明昔日淡水扼守台灣北部的險要地位。