Question: In which way were the walls of the Forbidden City built for the battle?
About the article "Forbidden City", section "Walls and gates".

Candidate passages:
Several enclosure walls remain, such as those built at the time of the citadel and the fort.
The "Citadel Walls" refers to the walls of the fort, which is laid out in the north-south direction and is fortified with many bastions.
It has four gates in good condition, which originally were double-storied, with rooms for housing the watch and guard staff.
The "City Walls at the Southeast Corner of the Citadel going up the hills" refers to the fort wall, built in ashlar stone masonry; four of its bastions are in good shape.
The "Walls of fort on top" refers to the walls of the fort, dated to approximately the 13th century, built in ashlar masonry. 
The Outer city was also called "Guocheng" or "Waiguo".
It was expanded in 1553.
The walls were 7.5 to 8 metres high, and were 12 metres wide at the bottom and 9 metres wide at the top.
The southeastern corner was built on the diagonal to avoid low swamps in the area.
This fulfilled the legend of Nüwa mending the heavens: the heavens were missing in the northwest and the earth was sinking in the southeast.
The southern walls encompassed the southern walls of the Temple of Heaven, and the eastern and western walls were parallel with the Inner city's walls, about 2 kilometres away.
This was where the rammed earth walls of the Yuan city of Dadu had been located before being abandoned in the closing years of the Hongwu era (1368–1398) during the early Ming dynasty.
Walls in the northeastern and northwestern sections were built using materials recovered from the Yuan rammed earth structure.
The Outer city walls had a perimeter of approximately 28 kilometres.
The entire enclosure of the inner and outer cities formed a "凸" shape with a perimeter of 60 kilometres.
Eleven gates were built, three each on the eastern, southern, and western walls, and two on the northern walls (one on the northeastern section and one on the northwestern section).
When the expansion of the Outer city was being planned in 1550, one idea called for nine small minor cities to be built around the nine Inner city gates, which would have been a formidable addition to the city defenses.
Construction began in 1550 on three of these minor cities.
However, because of the proximity to the Inner city gates, where many commoner's houses and shops had sprung up, many buildings needed to be dismantled.
The disgruntled commoners forced the abandonment of the project not long after construction began.
In 1553, it was decided that the remaining rammed earth walls of the Yuan city of Dadu would be dismantled and the raw materials used to complete the Outer city walls to form a "回": a square within a square.
The plan called for the Outer city walls to have a perimeter of 70 kilometres.
The east and west walls would have been 17 kilometres and the north and south 18 kilometres.
The plan called for 11 more gates, 176 more enemy sight towers, two additional sluice gates outside of Xizhimen and Tonghui River, and eight more sluice gates at other low swampy areas.
The Outer city expansion was a grand project, greater than any of the capitals of the previous Chinese dynasties.
The Emperor approved, and construction began in 1554.
Under the supervision of Yan Song, the project was divided into segments.
The southern walls were built first to add extra defenses for the bustling commercial district south of the Inner city gate at Zhengyangmen.
The eastern and western walls were planned for the second segment of the project.
However, due to successive invasions from the Mongols and Manchus, soldiers were called away to the northern frontiers to defend the Great Wall, and thus few could be spared for construction.
A great section of the Forbidden city caught fire in 1557, and workers and funding were funnelled towards repairs and reconstruction of the palaces.
The expansion of the Outer city walls was thus never completed.
In 1564, the barbicans of the Outer city gates were built. 
During Tang, the main exterior walls of Chang'an rose high, were by six miles in length, and formed a city in a rectangular shape, with an inner surface area of .
The areas to the north that jutted out like appendages from the main wall were the West Park, the smaller East Park, and the Daming Palace, while the southeasternmost extremity of the main wall was built around the Serpentine River Park that jutted out as well.
The West Park walled off and connected to the West Palace (guarded behind the main exterior wall) by three gates in the north, the walled-off enclosure of the Daming Palace connected by three gates in the northeast, the walled-off East Park led in by one gate in the northeast, and the Serpentine River Park in the southeast was simply walled off by the main exterior wall, and open without gated enclosures facing the southeasternmost city blocks.
There was a Forbidden Park to the northwest outside of the city, where there was a cherry orchard, a Pear Garden, a vineyard, and fields for playing popular sports such as horse polo and "cuju" (ancient Chinese football).
On the northwest section of the main outer wall there were three gates leading out to the Forbidden Park, three gates along the western section of the main outer wall, three gates along the southern section of the main outer wall, and three gates along the eastern section of the main outer wall.
Although the city had many different streets and roads passing between the wards, city blocks, and buildings, there were distinct major roads (lined up with the nine gates of the western, southern, and eastern walls of the city) that were much wider avenues than the others.
There were six of these major roads that divided the city into nine distinct gridded sectors (listed below by cardinal direction).
The narrowest of these streets were wide, those terminating at the gates of the outer walls being wide, and the largest of all, the Imperial Way that stretched from the central southern gate all the way to the Administrative City and West Palace in the north, was wide.
Streets and roads of these widths allowed for efficient fire breaks in the city of Chang'an.
For example, in 843, a large fire consumed 4,000 homes, warehouses, and other buildings in the East Market, yet the rest of the city was at a safe distance from the blaze (which was largely quarantined in East Central Chang'an).
The citizens of Chang'an were also pleased with the government once the imperial court ordered the planting of fruit trees along all of the avenues of the city in 740. 
In June 1802 Nguyễn Phúc Ánh took control of Vietnam and proclaimed himself Emperor Gia Long.
His rule was recognized by China in 1804.
Gia Long consulted with geomancers to decide which was the best place for a new palace and citadel to be built.
After the geomancers had decided on a suitable site in Huế, building began in 1804.
Thousands of workers were ordered to produce a wall and moat, 10 kilometers long.
Initially the walls were earthen, but later these earthen walls were replaced by stone walls, 2 meters thick.
The citadel was oriented to face the Huong River to the east.
This was different from the Forbidden City in Beijing, which faces south.
The Emperor's palace is on the east side of the citadel, nearest the river.
A second set of walls and a second moat was constructed around the Emperor's palace.
Many more palaces and gates and courtyards and gardens were subsequently added.
The rule of the last Vietnamese Emperor lasted until the mid-1900s.
At the time, the Purple Forbidden City had many buildings and hundreds of rooms.
It suffered from termite and cyclone damage, but was still very impressive.
Many bullet holes left over from the war can be observed on the stone walls.
In the early morning hours of January 31, 1968, as part of the Tet Offensive a Division-sized force of North Vietnamese Army and Viet Cong soldiers launched a coordinated attack on Huế seizing most of the city.
During the initial phases of the Battle of Hue, due to Huế's religious and cultural status, Allied forces were ordered not to bomb or shell the city, for fear of destroying the historic structures; but as casualties mounted in the house-to-house fighting these restrictions were progressively lifted and the fighting caused substantial damage to the Imperial City.
Out of 160 buildings only 10 major sites remain because of the battle, such as the Thái Hòa and Cần Thanh temples, Thế Miếu, and Hiển Lâm Các.
The city was made a UNESCO site in 1993.
The buildings that still remain are being restored and preserved.
The latest and so far the largest restoration project is planned to conclude in 2015. 
Kuai Xiang (; 1398–1481) was a Chinese architect and engineer widely known as designer of the Forbidden City and originator of the Xiangshan Carpenters.
He was born in Xukou (Xiangshan), Wu County, Suzhou during the Ming Dynasty.
When the Yongle Emperor decided to transfer the capital from Nanjing to Beijing in 1407, Kuai Xiang was ordered to design and construct the Forbidden City for him.
Kuai Xiang was in his early thirties when the Emperor commissioned him to design the Forbidden City.
Kuai used the Imperial Palace in Nanjing as a model.
He included features of palaces built during the Tang and Song Dynasties into his design.
He also drew from Confucian, Daoist, and traditional astronomical systems.
This took Kuai several years, and the first palace and walls of the City were completed only after 13 years.