This place would rate "very good" if it didn't smell a little too much of the sewer. I was there after seeing the castle last July, and it was a hot day. The kallaren beckoned due to the coolness of the underground and thick rock walls, all of which are very evocative of the castle's era. So you think to yourself, lovely, I am discovering this great little authentic Swedish restaurant. Which we did. The food is very good, and I could tell it was popular with tourists, since I could hear English being spoken around me, definitely not the norm in Sweden.
However, in spite of how good the sköttbullar in grädfil are, the place is underground, and in the summer, it does smell a little too much of the sewer. Perhaps in the winter it doesn't smell as bad.
If you can get past the slight smell of the sewer, it's a nice restaurant, it has nice table settings with candles, and it has great, intimate stone walls and a romantic atmosphere. It is more expensive, probably, and there are many other restaurants in Kalmar, but the cellar is one of those places that makes travelers feel as though they have seen "authentic" Sweden, and eaten authentic cuisine, which is true, but it's going to be somewhat more expensive and it's going to smell. I know this makes it sound bad, but it's a good restaurant, just not in summer, probably.
Because of the choices we're given here, to rate it between 'very good' and 'average,' I am having a hard time and feel like I have to explain to the reader that a unique restaurant like this could not be considered average, but because of the issues like stench and cost, it's probably not going to be considered "very good" by everyone, although it should be.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.