Plot-holes can damage brains...

Looking at the now-dead Ogre's corpse Rio didn't know what he should do now. According to what he read in the novel there shouldn't be any monster here, yet Rio met this damn ogre here. 

Not to mention the fact that ogres are rather rare and can only be found in very few dungeons, they are also a race with intelligence, although their IQ is still low than an average human, they are still capable of carrying out complex movements like using heavy swords and battleaxes in a skillful way.

Thinking about the event of the Arte's tower in the original novel it was as follows:

Fade and Kira chose this tower for their date. While walking around Kira told Fade the whole history of the tower, as you know she is very good with this aspect. She has deep knowledge of history and generally, she always likes to read history books whenever she has free time.