The longest city walls were those of Classical Athens. Their extraordinary length was due to the construction of the famous Long Walls which played a key role in the city's maritime strategy, by providing it with a secure access to the sea and offering the population of Attica a retreat zone in case of foreign invasions. At the eve of the Peloponnesian War (431–404 BC), Thucydides gave the length of the entire circuit as follows: 43 stades (7.6 km) for the city walls without the southwestern section covered by others walls and 60 stades (10.6 km) for the circumference of the Peiraeus port. A corridor between these two was established by the northern Long Wall (40 stades or 7.1 km) and the Phaleric Wall (35 stades or 6.2 km). Assuming a value of 177.6 m for one Attic stade, the overall length of the walls of Athens thus measured about 31.6 km. The structure, consisting of sun-dried bricks built on a foundation of limestone blocks, was dismantled after Athens’ defeat in 404 BC, but rebuilt a decade later. Syracuse, Rome (Aurelian Walls) and Constantinople (Walls of Constantinople) were also protected by very long circuit walls.
The largest monolith lifted by a single crane can be determined from the number of holes (each of which points at the use of one crane) in the lifted stone block. By dividing its weight by their number, one arrives at a maximum lifting capacity of 7.5 to 8 t as exemplified by a cornice block at the Trajan's Forum and the architrave blocks of the Temple of Jupiter at Baalbek. Based on a detailed Roman relief of a construction crane, the engineer O'Connor calculates a slightly less lifting capability, 6.2 t, for such type of a crane, on the assumption that it was powered by five men and using a three-pulley block.
The largest monolith lifted by cranes was the 108 t heavy corner cornice block of the Jupiter temple at Baalbek, followed by an architrave block weighing 63 t, both of which were raised to a height of about 19 m. The capital block of Trajan's Column, with a weight of 53.3 t, was even lifted to 34 m above the ground. As such enormous loads far exceeded the lifting capability of any single treadwheel crane, it is assumed that Roman engineers set up a four-masted lifting tower in the midst of which the stone blocks were vertically raised by the means of capstans placed on the ground around it.
The largest monoliths were two giant building blocks in the quarry of Baalbek: an unnamed rectangular block which was only recently discovered is measured at 20 m x 4.45 m x 4.5 m, yielding a weight of 1,242 t. The similarly shaped Stone of the Pregnant Woman nearby weights 1,000.12 t. Both blocks were intended for the Roman temple nearby, but were left for unknown reasons at their sites.
The largest monolith was a group of three monumental blocks in the podium of the Jupiter temple at Baalbek. The individual stones are 19.60 m, 19.30 m and 19.10 m long respectively, with a depth of 3.65 m and a height of 4.34 m. Weighing approximately 800 t on average, they were transported to a distance of 800 m and probably pulled by the means of ropes and capstans into their final position. The supporting stone layer beneath features a number of blocks which are still in the order of 350 t. The various giant stones of Roman Baalbek rank high among the largest man-made monoliths in history.
The largest monolithic columns were used by Roman builders who preferred them over the stacked drums typical of classical Greek architecture. The logistics and technology involved in the transport and erection of extra-large single-piece columns were demanding: As a rule of thumb, in the length range between 40 and 60 Roman feet (approximately 11.8 to 17.8 m), the weight of the column shafts, due to their larger diameter, doubled with every ten feet from 50 over 100 to 200 t. Despite this, forty and also fifty feet tall monolithic shafts can be found in a number of Roman buildings, but examples reaching sixty feet are only in evidence in two unfinished granite columns which still lie in the Roman quarry of Mons Claudianus, Egypt. One of the pair, which was discovered only in the 1930s, has an estimated weight of 207 t. All these dimensions, however, are surpassed by Pompey's Pillar, a free-standing victory column erected in Alexandria in 297 AD: measuring 20.46 m high with a diameter of 2.71 m at its base, the weight of its granite shaft has been put at 285 t.
The largest monolithic dome crowned the early 6th century AD Mausoleum of Theodoric in Ravenna, then capital of the Ostrogothic kingdom. The weight of the single, 10.76 m wide roof slab has been calculated at 230 t.
The longest trackway was the Diolkos near Corinth, Greece, measuring between 6 and 8.5 km. The paved roadway allowed boats to be pulled across the Isthmus of Corinth, thus avoiding the long and dangerous sea trip around the Peloponnese peninsula. Working by the railway principle, with a gauge of around 160 cm between two parallel grooves cut into the limestone paving, it remained in regular and frequent service for at least 650 years. By comparison, the world's first overland wagonway, the Wollaton Wagonway of 1604, ran for 3 km.
The largest prop-and-lintel roof by span spanned the Parthenon in Athens. It measured 19.20 m between the cella walls, with an unsupported span of 11.05 m between the interior colonnades. Sicilian temples of the time featured slightly larger cross sections, but these may have been covered by truss roofs instead.
The largest truss roof by span covered the Aula Regia (throne room) built for emperor Domitian (81–96 AD) on the Palatine Hill, Rome. The timber truss roof had a width of 31.67 m, slightly surpassing the postulated limit of 30 m for Roman roof constructions. Tie-beam trusses allowed for much larger spans than the older prop-and-lintel system and even concrete vaulting: Nine out of the ten largest rectangular spaces in Roman architecture were bridged this way, the only exception being the groin vaulted Basilica of Maxentius.
The deepest tunnel was the Claudius Tunnel, constructed in eleven years time by emperor Claudius (41–54 AD). Draining the Fucine Lake, the largest Italian inland water, 100 km east of Rome, it is widely deemed as the most ambitious Roman tunnel project as it stretched ancient technology to its limits. The 5653 m long tunnel, passing under Monte Salviano, features vertical shafts up to 122 m depth; even longer ones were run obliquely through the rock. After repairs under Trajan and Hadrian, the Claudius tunnel remained in use until the end of antiquity. Various attempts at restoration succeeded only in the late 19th century.
The longest road tunnel was the Cocceius Tunnel near Naples, Italy, which connected Cumae with the base of the Roman fleet, Portus Julius. The 1000 m long tunnel was part of an extensive underground network which facilitated troop movements between the various Roman facilities in the volcanic area. Built by the architect Cocceius Auctus, it featured paved access roads and well-built mouthes. Other road tunnels include the Crypta Neapolitana to Pozzuoli (750 m long, 3–4 m wide and 3–5 m high), and the similarly sized Grotta di Seiano.
The longest tunnel as well was the 94 km long Gadara Aqueduct in northern Jordan. This recently discovered structure provided for hundreds of years water for Adraa, Abila and Gadara, three cities of the ancient Decapolis. Only 35 km long as the crow flies, its length was almost tripled by following closely the contours of the local topography, avoiding valleys and mountain ridges alike. The monumental work seemed to be carried out in seven stages of construction between 130 and 193 AD. The distance between the individual vertical shafts was on average 50 m. Probably the project was initiated by Hadrian, who had granted privileges to the cities during a longer stay in the Decapolis. The aqueduct remained operational until the Byzantines lost control of the region after the Battle of Yarmuk in 636.
The longest tunnel excavated from opposite ends was built around the end of the 6th century BC for draining and regulating Lake Nemi, Italy. Measuring 1600 m, it was almost 600 m longer than the slightly older Tunnel of Eupalinos on the isle of Samos, the first tunnel in history to be excavated from two ends with a methodical approach. The Albano Tunnel, also in central Italy, reaches a length of 1400 m. It was excavated no later than 397 BC and is still in service. Determining the tunnelling direction underground and coordinating the advance of the separate work parties made meticulous surveying and execution on the part of the ancient engineers necessary.
The largest barrel vault by span covered the Temple of Venus and Roma, Rome. Built between 307 and 312 AD, the vaulted structure replaced the original wooden truss roof from Hadrian’s time.
The largest groin vault by span roofed the 25.01 m wide main nave of the Basilica of Maxentius on the Forum Romanum, built in the early 4th century.