Ottoman Empire, Gallipoli Star on Screwback. Type Godet.

German manufacture and apart from the enamel damage to the centre(!), in a good, worn condition.
Not marked but a known variation that has been attributed to the company of Godet & Co. from Berlin. Fine, multi-piece construction with pierced out Tughra and date and all attached with small rivets.
Very unusual and difficult to find screwback variant.