Which preposition to use with illius

in Occurrences 2%

Quia circa medium illius in summo apice turris maioris, duo sunt nodi seu pomella de decoctissimi auri metallo miræ magnitudinis, et serenæ resplendentiæ, et in ipsis formati duo carbunculi grandes, et lati, sua virtute tenebras effugantes, et velut splendorem plenilunij nocturno tempore mentientes.

Which preposition to use with  illius