Short answers, pulled from the story.
Neoclassical architecture is an architectural style that emerged from the Neoclassical movement starting in the mid-18th century in Italy, France, and Germany. It aimed to strip away the excesses of Late Baroque and return to a purer, more authentic form of classical Greek and Roman design. The shift is conventionally dated to the 1750s.
Neoclassical architecture emphasizes the wall rather than chiaroscuro, maintains separate identities for each of its parts, and is defined by symmetry, simple geometry, and flat sculptural ornament. It reacted against the naturalistic ornamentation of the Rococo style, favoring planar surfaces, bas-relief friezes, and restrained decoration.
Key figures include Karl Friedrich Schinkel in Prussia, Sir John Soane and Robert Adam in Britain, Benjamin Henry Latrobe and Thomas Jefferson in the United States, Juan de Villanueva in Spain, and Theophil Hansen in Greece. In France, the court architects Percier and Fontaine shaped the Empire style under Napoleon I.
Excavations at Pompeii and Herculaneum, begun in the late 1740s, revealed that earlier Baroque interiors had been based on temple and basilica exteriors turned inside out. The illustrated volumes of Le Antichità di Ercolano Esposte, which reached a wide audience in the 1760s, inspired a new interior vocabulary using flat relief motifs, medallions, and colors such as Pompeiian red.
Greek Revival was a second wave of neoclassical architecture that gained momentum from about 1800, drawing on a fresh influx of Greek architectural examples seen through etchings and engravings. It was more severe and consciously archaeological than the first wave, which had been primarily based on Roman models. The Society of Dilettanti funded the first serious study of Greek buildings in 1751, led by James Stuart and Nicholas Revett.
Notable planned cities that reflect neoclassical urban thinking include Karlsruhe, Washington D.C., Saint Petersburg, Buenos Aires, Havana, and Barcelona. The style drew on the Roman grid system of streets, a central forum, and boulevards scaled to the importance of public buildings.