Dempo Mausoleum
The barred tomb your father entered with a company of knights, and the wall the demo keeps you on the outside of.
The mausoleum is the demo’s horizon. You can reach the door, read the inscription, and be told plainly that the company of knights went in and never came back out, but you cannot follow them. Everything the demo does well is arranged around that refusal.
Instead of an entrance, you get the search for one. A guard on the far side of the island asks for a key, a burial site is named as the place that key is kept, and the shrine gate it actually opens turns out to be a different door entirely. That misdirection is the demo’s whole structural joke, and it is a good one.
Worth saying for anyone planning a purchase: the free demo is a slice of the opening area, not a shortened version of the full game. What you are judging is whether the loop of shore, forest, crypt and gate is the loop you want for dozens of hours.