This is a powerful and somber work, originally conceived for mechanical organ. The fugue subject is exceptionally dark and brooding, demanding a precise and controlled execution.
The development of independent contrapuntal lines while maintaining rhythmic drive is a significant challenge.
Adagio and Fugue in C minor (K.546) is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7. In the RCM system it corresponds to about level 7.
You should be comfortable at Henle level 5 — meaning you can already play level 4–5 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Adagio and Fugue in C minor (K.546) lasts about 6 minutes.
Adagio and Fugue in C minor (K.546) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.