This is a somber, commemorative piece written in the tradition of funeral marches. It employs heavy, sonorous chords and a slow, measured tempo to achieve its solemn gravity.
The primary challenge is maintaining an even, weighted touch for the heavy chordal progressions without creating a harsh sound.
Comparable difficulty and character — good candidates for what to learn next.
Au soldat inconnu (Op.184) 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 Au soldat inconnu (Op.184) lasts about 5 minutes.
Au soldat inconnu (Op.184) 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 'The King of the Jews' - Incidental Music by Alexander Glazunov, Bäckahästen by Kurt Atterberg, Alleluia by Charles-Valentin Alkan — see the full similar-pieces list on this page.