During the Assyrian Empire's historical span from the 25th century BCE to 612 BCE, architectural styles went through noticeable changes. Assyrian architects were initially influenced by previous forms dominant in Sumer and Akkad. However, Assyrian structures eventually evolved into their own unique style. 

Temples

Little is known of the construction of Assyrian temples with the exception of the distinctive ziggurats and massive remains at Mugheir. Ziggurats in the Assyrian Empire came to be built with two towers (as opposed to the single central tower of previous styles) and decorated with colored enameled tiles. Contemporaneous inscriptions and reliefs describe and depict structures with octagonal and circular domes, unique architectural systems for the time. Little remains of the temple at Mugheir, but the ruins of its base remain quite impressive, measuring 198 feet (60 m) long by 133 feet (41 m) wide by 70 feet (21 m) high.

Dur-Sharrukin

Building plans remained rectangular through much of the empire's history. The fortress of Sargon II (reigned 722–705 BCE) at Dur-Sharrukin, or Khorsabad, was the best known. Consisting of a stone foundation punctuated by seven gates, the fortress housed the emperor's palace and a ziggurat among massive load-bearing walls with regularly spaced towers. 

Reconstruction of the palace of Sargon II at Dur-Sharrukin

Image shows the typical rectangular plan and massive fortified walls of Assyrian palace architecture. 

Despite the intended political symbolism of Assyrian superiority, these fortified walls signify preparation for an attack by enemy invaders. Among the ornamental features excavated was a monumental lamassu outside the throne room. After the death of Sargon II, the site was abandoned.

Lamassu from Dur-Sharrukin

Nimrud

Lamassu figures abounded throughout the Assyrian Empire, featuring in the Northwest Palace of Ashurnasirpal II (reigned 883-859 BCE) at Nimrud. Reconstructions show that they adorned the gateways of the palace, including an entrance marked by a round arch. According to contemporaneous inscriptions, the palace consisted of wood from a diverse number of tree species, alabaster, limestone, and a variety of precious metals. As with Dur-Sharrukin, the palace of Ashurnasirpal II was surrounded by fortified load-bearing walls.

Balawat Gates

Builders increasingly used wood, particularly cedar and cypress, in architecture. As a result, much of the architecture has decayed, leaving archaeologists to produce reconstructions for present-day scholars. One example is the Balawat Gates, from the Assyrian outpost of Balawat, or Imgur-Enlil. Two sets were commissioned during the reign of Ashurnasirpal II and one addition set under the reign of his son Shalmaneser III (859–824 BCE). Assyrian inscriptions suggest the gates were made of cedar. Experts estimate that the gates stood over 22 feet high. The metal bands that adorned the gates suggest that they measured 285 feet wide. Lacking hinges, the gates opened by turning enormous pine pillars that rotated in stone sockets. Despite the long-term fragility of wood, the scale of the gates and the mechanisms by which they opened and closed point to the political instability of the time and the need to defend all parts of the empire.

Reconstruction of the Balawat Gates at the British Museum

Woman in photograph provides an idea of the scale of the gates.