He probably had financial success mainly with portraits, e.g. of the German Emperor Wilhelm I or Otto von Bismarck's for local and public clients.
In addition, he also produced "surreal" compositions, such as Größenwahn (1891), Aschermittwoch (1892), or Der Clown, an allegory of "contemporary artistic excesses" (1892), as well as landscape depictions and numerous other portraits.
Among other things, Daelen wrote art criticism, which appeared under the pseudonyms Ursos teutonicus and Angelo Dämon in various papers.
Ein Buch für Künstler und Ärzte, with 336 artistic nude studies after photographic images (with the collaboration of Dr. Gustav Fritsch, Professor of Anatomy at the University of Berlin and J. Paar), Stuttgart, Hermann Schmidt 1907.
The pious Helene, which in today's view caricatures above all religious hypocrisy and dubious bourgeois morality, Daelen saw as an attack on "female mischievousness, curiosity and vanity, as well as the always speculative sophistication despite all laziness and narrow-mindedness".
The literary scholar Friedrich Theodor Vischer, who in his essay Über neuere deutsche Karikatur (On Recent German Caricature) found a respectful appreciation of Busch as well as some critical remarks, attacked Daelen in page-long tirades as a "literary bonze" and accused him of the "eunuch envy of the dried-up philistine".
His essay, which appeared in the Frankfurter Zeitung, contained a number of incorrect biographical data and was the occasion for Wilhelm Busch to comment on his person in the same newspaper.
He also found the treatment of his relationship with Johanna Keßler, who had strongly supported him in his Frankfurt years, indiscreet and tastelessly illuminated.