Mafra

Of note, the Palace of Mafra is known to have the longest hallway of all the European palaces. The rest of this narrative is from Wikipedia with information considerably less useless.
The palace, which also served as a Franciscan friary, was built during the reign of King John V (1717-1750), as consequence of a vow the king made in 1711, to build a convent if his wife, Maria Anna of Austria,[2] gave him offspring. The birth of his first daughter the Infanta Barbara of Portugal, prompted construction of the palace to begin. The palace was conveniently located near royal hunting preserves, and was usually a secondary residence for the royal family.
The last king of Portugal, Manuel II, following the proclamation of the republic, left on 5 October 1910 from the palace to the nearby coastal village of Ericeira on his way to exile. The palace was declared a national monument in 1907.
The Portuguese royal court transferred from Lisbon to the Portuguese colony of Brazil in a strategic retreat of Queen Maria I of Portugal, prince regent John, the Braganza royal family, its court, and senior officials, totaling nearly 10,000 people, on 27 November 1807.The embarkment took place on 27 November, but due to weather conditions, the ships were only able to depart on 29 November. The Braganza royal family departed for Brazil just days before Napoleonic forces invaded Portugal on 1 December 1807. The Portuguese crown remained in Brazil from 1808 until the Liberal Revolution of 1820 led to the return of John VI of Portugal on 26 April 1821.
In 1807, at the outset of the Peninsular War, Napoleonic forces invaded Portugal due to the Portuguese alliance with Great Britain. The prince regent of Portugal at the time, John of Braganza, had formally governed the country on behalf of his mother, Queen Maria I of Portugal, since 1799. Anticipating the French invasion, John ordered the transfer of the Portuguese royal court to Brazil before he could be deposed. Setting sail for Brazil on 29 November 1807, the royal party navigated under the protection of the British Royal Navy, and eight ships of the line, five frigates, and four smaller vessels of the Portuguese Navy, under the command of Rear-Admiral Sir Sidney Smith. On 5 December, almost halfway between Lisbon and Madeira, Sidney Smith, along with Britain's envoy to Lisbon, Percy Smythe, 6th Viscount Strangford, returned to Europe with part of the British flotilla. Commodore Graham Moore continued escorting the Portuguese royal family to Brazil with the British ships Marlborough, London, Bedford, and Monarch.
For thirteen years, Rio de Janeiro functioned as the capital of the United Kingdom of Portugal, Brazil and the Algarves.

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Stunning

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Lisbon's largest building.