4 KudoZ points awarded for this answer. Comment: "The most helpful approach because the (exact, architectural) context has shifted ground while you/we've been pondering, and still isn't 100% clear. If the author's "Turmhalle" and where the artist says the author saw the work - a "Turmkapelle" - coincide, it probably won't be a vestibule, tho' interior, nor the narthex - can't envisage a chapel as a place of passage. Chamber sounds too enclosed , tho' chapels within (the trad. notions of) churches are, on at least three sides... so I'm mugwumping and opting for 'the chapel space beneath the tower' with vestibule and narthex as options should they apply - something the author and artist will know, given your info. The deadline won't allow me to ask for pics of the church in qn. Vielen Dank an alle fürs Mitdenken! and the helpful URLs."