以内 within; within the limits of; including; less than 长城以内 within the great wall; 在三天以内 within three days; 限制在五十人以内 within the limits of fifty people; 这种药三年以内有效 。 this medicine is effective if used within three years
长城 1.(中国的万里长城) the great wall2.(坚不可摧的堡垒) impregnable bulwark