splendid multistoried building; magnificent pavilion
Refers to an ornate, multi-story structure, often in classical Chinese-influenced literature. The cross-reference to 楼閣 suggests a similar meaning but with a more poetic or elevated tone.
See also: 楼閣
In the poem, the 鳳閣 is depicted as a magnificent building towering above the clouds.
Ministry of Central Affairs (ritsuryō system)
Historical government office under the ritsuryō system. This meaning is highly specialized and only encountered in texts about ancient Japanese bureaucracy.
The 鳳閣 was one of the government offices responsible for central administration under the ritsuryō system.