Ludwig’s reign was from 1825 to 1848 and it was constructed during it, but the idea predates it. https://en.wikipedia.org/wiki/Walhalla_(memorial)

“It was conceived in 1807 by Crown Prince Ludwig in order to support the gathering momentum for the unification of the many German states. Following his accession to the throne of Bavaria, construction took place between 1830 and 1842 under the supervision of the architect Leo von Klenze”