The author surveys the data from written sources concerning the public works obligation imposed by the Piast monarchy in order to build fortifications (strongholds and entanglements). The origin of public works is usually traced back to the tribal tradition, strengthened by adopting the Western principle of the monarch's control over fortifications. Due to the scarcity of preserved sources and their character it is difficult to establish when exactly the obligations known from the 13th c. were introduced. The type of the available sources also limits our knowledge of the scope of fortification obligations. Almost all the mentions of fortification obligations come from documents granting jurisdiction privileges to the Church, therefore we know very little of the obligations imposed on knights. The sources record not only the names of particular fortification works but also some details that allow us to hypothesise about the method of their execution and the authorities responsible for enforcing the fulfilment of the obligation. The fortification obligations included: cutting trees to build an entanglement or barricade (preseka, incisurae, precisio nemorum) (building an entanglement in wartime was considered a separate category), the construction and repair of strongholds, and breaking the ice in moats surrounding strongholds (naramb, narub). The most frequently mentioned obligations were those connected with the building and repairing of strongholds. Building new strongholds and repairing old ones were considered separate categories of works. In addition to the most common names of works, there are less frequent terms referring to various extra duties (digging moats, building and repairing bridges leading to strongholds, covering the surface of stronghold walls with pug). There was a range of Latin terms used to refer to fortification works: form the simple expression labor/opus castri through the general terms constructio or edificatio to names indicating the variety of works connected with fortification construction (reedificatio, reformatio, munitio, reparatio, correctio, firmatio). The only native name of an obligation is the word grod ('stronghold') recorded in two documents. The obligation to participate in building strongholds obviously concerned both serfs and knights, the latter group, however, to a limited extent. A knight was obliged to build one 'box' (section) of the stronghold wall. Exemptions from fortification works granted by princes to serfs from Church estates indicate that the priority for the rulers of Polish principalities in the 13th c. was the upkeep of their capitals and borderland strongholds. It is difficult to figure how time- and labour consuming fortification works were, although they were certainly burdensome. The fulfilment of the obligation was almost certainly not supervised by castellans; some documents suggest that there were special officials appointed for that task. There are no data on serfs being engaged in fortification works. There is no information on the logistics and organization of the works, either. The obligations known from 13th-century sources were gradually abandoned due to the extension of jurisdiction priviledges; they were abolished by the general privileges granted to the Kingdom of Poland in 1374 and to Mazovia in 1418 and 1478. Their extinction could also have been connected with the replacement of wooden strongholds by brick castles. The building of entanglements was probably given up because of the intensive colonization of forest areas.