Can I use “as” to replace the “when”？
Solution 1:
It's more common to use as with an action. "He was fiddling as Rome burned" is fine; "He was in Tehran as Rome burned" is a little awkward. (That's in my American dialect. YMMV.) So I wouldn't use as in your sentence.
Also, use was in place of your alwas, and put a space between St. and Peter's.