The Citadel

Poul Anderson mentions the Citadel in The Enemy Stars, then describes it in "The Ways of Love." This is good future historical writing even though only in a single novel and short story, set only a decade apart.

The Citadel is an Alpine fortress, "...remodeled and enlarged to hold the masters of the globe..." ("The Ways of Love," see here.)

From a parapet, the visiting Arvelian couple see:

steeps and cliffs tumbling into a valley;
heights beyond;
a waterfall;
white peaks.

The narrator explains that, on Earth, water often freezes and that the sun appears slightly larger than Sarnir. Wind booms hollowly while his wife non-verbally expresses her wish to experience the Terrestrial environment directly without having to wear a spacesuit to protect her from its oxygen.