The wall and redoubts, built by the Prussians so they could better besiege the city, is behind the soldier.
The Danes retained Schleswig in the first Battle of Schleswig Holstein 1848 or so, where the sides were roughly evenly matched; but lost it in the Second Battle of Schleswig about 1850 when the German Federation joined forces with Schlewig-Holstein. The Danes call these their civil wars; the Germans may call them wars of secession.
The Danes were vastly outnumbered in 1864, more matched in 1848. See Danish military history at ://www.milhist.dk/start/uk_slesvigwars.htm/. The Fredericia battle is at ://www.milhist.dk/trearskrigen/fredericia/fredericia_uk.htm/. These wars recur, so it helps to learn the basics early on. Search also for the spelling Slesvig".
The Battle of 1864, Denmark against Prussia and Austria: focus there is at the Sonderborg post, a headland fortification, town and castle. There the Danes lost, but celebrate the courage of the outnumbered Danish soldiers, see Sonderborg post.
The "Great Barrow" found in searches refers to the mass grave with a mound on top. Some 500 soldiers' remains are at the great barrow at Trinity? It looked too small for that, still checking.