He established the fortress of Passavant or Passava—the name probably derives from the war-cry or family motto "passe avant", but is also found as a toponym in northeastern France—on the mountains between the Mani peninsula and the plain of Laconia.

[3] His barony probably comprised newly conquered land—it was not until c. 1248, with the fall of the last Byzantine fortress, Monemvasia, that Laconia was fully pacified—with four knight's fiefs.

It was militarily important, since it kept watch over the unruly Maniots and the Slavic inhabitants of Mount Taygetos, and Nully was named hereditary marshal of Achaea.

[6] To fill the period between its foundation and the 1260s, the historian Karl Hopf hypothesized that there were two barons of Passavant, both named John, but this conjecture was rejected by Antoine Bon.

In the meantime, Passavant had fallen to the Byzantines during their first offensives in the Peloponnese, c. 1263, and the Barony of Akova was confiscated by the Prince because Margaret delayed too long in claiming her inheritance.