Karl Mach's compositions are rare, generally reflecting the aesthetic of early 20th-century central European salon music. The piece exhibits a simple melodic structure supported by gentle harmonic movement.
Pay close attention to the voicing of the inner lines within the chordal textures.
Comparable difficulty and character — good candidates for what to learn next.
Allein is rated Henle level 3 of 9 — late beginner repertoire, roughly ABRSM grades 4–5. In the RCM system it corresponds to about level 4.
You should be comfortable at Henle level 3 — meaning you can already play level 2–3 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 Allein lasts about 2 minutes.
Allein 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 Für Elise by Ludwig van Beethoven, Prelude in E minor, No. 4 by Frédéric Chopin, As in a Rose Jar by Charles Wakefield Cadman — see the full similar-pieces list on this page.