Kirk here. I didn’t know what romon (楼門; ろうもん) was so I looked it up. The first character means “tall building” and the second means “gate” so it’s a majestic gate of the sort you see in the picture. Here’s the link to the article and a YouTube video:
https://www.asahi.com/ajw/articles/14949203
P.S. The preview image generated by the article was unrelated to Aso Shrine so I used a screenshot instead.