Tower Dungeon 4 by Tsutomu Nihei

Tsutomu Nihei, master of the mysterious architectural monolith, brings his genius to bear on the fantasy genre for the first time. Having established himself with hit series like Blame! and Knights of Sidonia, he has now created a gritty, dark fantasy world and populated it with evil sorcerors and unique undead horrors.
The ingenuous farmboy Yuva must accompany a contingent of knights through the increasingly dangerous floors of massive tower on the way to rescue the princess, Nihei bringing his unique sensibility to bear on a crumbling world of malformed creatures, wounded soldiers, and labyrinthine darkness.

Even as the mystery of Yuva’s lineage is unraveled, a new group of intrepid (and somewhat seasick) adventurers arrives to assay the Dragon Tower. A crippled dracomorph and his mute assistant, a Ratwick princess and her stout knight defender, an aged pyromancer and his loyal crow-goat steed, a battle maiden in full plate armor—what dangers await this motley crew within the labyrinthine stone monolith’s walls…?