Rigel's keyboard works reflect the shift from mid-18th-century galant style to earlier high-Classical experimentation. This work is highly dependent on light, detached articulation and clean scalar passages.
At Henle 6 this is an advanced piece that assumes fluent technique and rewards mature interpretation. The main demands: Alberti-bass accompaniment kept quiet and even under the melody; fast passage work that stays light and even at tempo; and precise articulation — the difference between crisp and blurred is the piece.
The technical challenge lies in the rapid, crisp execution of the accompaniment figures without sacrificing melodic character.
Comparable difficulty and character — good candidates for what to learn next.
A level or two down with the same spirit — ideal if this piece is still out of reach.
Where this piece's skills lead: slightly harder works making similar technical demands.
Fantasia (Op.21) is rated Henle level 6 of 9 — advanced repertoire, roughly ABRSM grades 7–8. In the RCM system it corresponds to about level 8.
You should be comfortable at Henle level 6 — meaning you can already play level 5–6 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 Fantasia (Op.21) lasts about 6 minutes.
Fantasia (Op.21) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include Etude in F minor by Felix Mendelssohn, Partita No. 1 in B-flat major by Johann Sebastian Bach, Bugatti Step by Jaroslav Ježek — see the full similar-pieces list on this page.