Okay, this is splitting hairs: the tutorial pop-up is talking about the golden symbol in the bottom left. But it calls the symbol “Roundtable icon,” which doesn’t help. This symbol pops up at quite a number of places and prevents you from killing NPCs in those areas. Still, not FromSoft’s best communication with the player.
You can actually fight in the Roundtable Hold before Ensha (the guy in the skeleton armor) invades: When you jump down from the balcony to the lower floor, you get invaded by Mad Tongue Alberich. (He is relatively easy). This area is useful after defeating him, when you want to try out weapon move sets.
Ensha is not guaranteed to invade you. He only invades if you pick up either of the Haligtree Medallion halves and only if you do it before defeating Fire Giant.
Another character that can also violate the supposed safety of the Roundtable Hold is Fia. In one version of her side quest, she kills D in the Roundtable Hold.
For people who are further interested in video games that violate safe rooms: here is a video essay by Daryl Talks Games about the topic.