Question

To see things on the other side of a high wall……………device
can be used

Answer 1

Answer-

I think its mirror.

Answer 2

Answer—

To see things on the other side of a high wall Periscope device can be used.

Explanation—

Periscope is a kind of instrument which is used to observe or see the object even though there is not a direct sight for the object or there is a presence of any obstacle.