We slip out of the postern door of the castle. 我们从城堡的後门溜出。
He sat within earshot, milking the cows by the light of a lantern, which I seized unceremoniously, and, calling out that I would send it back on the morrow, rushed to the nearest postern. 他坐在说话听得见的近处，借着一盏提灯的光在挤牛奶，我就毫无礼貌地把提灯抢过来，大喊着我明天把它送回来，便奔向最近的一个边门。
现代英汉综合大辞典
(城, 教堂等 的)后门, 旁门; 便门
柯林斯
百科