This book presents a little-known and ingenious artefact of the Roman world: a small puzzle padlock whose font plate bears a face or 'mask' of 'Celtic' style. The padlocks were designed to secure small bags or pouches and their distribution extended across Europe with the majority found in the Danubian region and in the vicinity of Aquileia. The authors examine the cultural context, the origins and uses of the padlocks, and provide detailed solutions to the puzzle mechanisms. The publication provides a fully-illustrated catalog of the known 156 examples, categorises their types according to construction and style, and explores the technicalities of the subject by the process of constructing replica mask puzzle padlocks.