Apollodorus of Damascus is the best-known architektôn of the early second century AD, the era of Trajan and Hadrian. In the civil domain he is credited with planning and constructing prestigious projects in Rome itself, including Trajan's Forum and Baths; in the military sphere he bridged the Danube and wrote a Siege-matters treatise for his patron-emperor. Addressed (it is argued here) to Trajan rather than Hadrian, and with a view to the campaigning conditions anticipated in Dacia, the treatise therefore proffered suggestions and designs suitable for a Roman army operating in that rugged terrain and attacking its hill-top settlements. However, as P. H. Blyth first realised, what has been transmitted under Apollodorus' name includes many later elaborations, armchair-fantasy inventions which, if ever built, could never have been effective. This, the work's first English translation and the first full commentary on it in any language, gives modern readers criteria for differentiating between these two disparate categories of material, thus allowing an assessment of each component in the terms appropriate to it.