。
He disguised himself as a farmer and infiltrated the castle town.
Literal
He [topic-は] farmer [to-に] appearance [object-を] disguising castle-town [to-に] infiltrated.
身をやつす ('to disguise oneself,' literally 'to let one's body become shabby/thin') is a literary expression for taking on a humble disguise. 農夫 ('farmer') is the cover identity. 城下町 (じょうかまち, 'castle town') is the historical term for the town that grew around a feudal lord's castle — immediately signals a period setting. 潜入した ('infiltrated') is the spy/military verb.