They served to defend the city, but they also kept out undesirable people

They served to defend the city, but they also kept out undesirable people, like those with contagious diseases.
serve: phục vụ cho việc gì / dùng cho việc gì
Các từ còn lại:
suppose: cho là, tin rằng
expect: trông đợi
mean: nghĩa là / có ý định