Self-styled valiant Alcasto (draft) or Drogo (final text) reaches the haunted forest, where however a "firewall" suddenly stops him.
Cresce il gran foco, e 'n forma d'alte mura
Stende le fiamme torbide e fumanti
E ne cinge quel bosco, e l'assecura
Ch'altri gli arbori suoi non tronchi o schianti.
Le maggiori sue fiamme hanno figura
Di castelli superbi e torreggianti;
E di machine ardenti anco ha munite
Le torri sue questa superba Dite.
The great fire grows, stretching its flames
In the shape of very high, smoking walls
That surround the whole wood and prevent
All from cutting or felling its trees.
Its most developed flames have the shapes
Of magnificent and towering castles,
While burning devices also defend
The towers of such a superb Dis.
The reference is to Dante's City of Dis in hell, see Inferno 9: 67-75, but here more technologically advanced according to the military developments in the Renaissance -- that is, the time period in which the poem was written, not the period in which it is set (late 11th century).
The Italian adjective superbo, and inflected forms, may mean both "superb, magnificent" and "proud," hinting at the devil.