In particular, it has a monolithic head and long-and-short work jambs. These techniques are typical of the Anglo-Saxon period and suggest that Kirtling church was built before the Norman Conquest.
This is interesting, because the Domesday Book (written in 1086) records that Kirtling had been an important place in Anglo-Saxon times, but it doesn’t mention the church. However, the Domesday Book is notoriously unreliable about churches and this window suggests that Kirtling was one of the churches its authors omitted.
'... the doorway is Romanesque or Norman in style ...'
Inside the porch, the doorway is Romanesque or Norman in style, and so dates to the 12th century. It has a tympanum depicting Christ seated on a throne, and several bands of chevron ornament. This throws up more questions about the history of the church.
The relationship between door and porch is the easiest to answer - the walls of the 15th-century porch slightly overlap the door surround, so the porch was clearly put on later. But how can the door be dated 100 or so years later than the little window? The answer seems to be that the door was intended to bring the old church up to date with the latest architectural fashions.
This ties in with the history of Kirtling castle, which came into the possession of the de Tosny family after the Conquest. It was they who fortified the raised mound site, and the work on the church would have been a statement of their power as Normans over the English people living in Kirtling.