Is there a "Peking Opera dialect"? Peking Opera has a distinct pronunciation, that is at first similar to Mandarin, but to astute listeners, certain characters appear to have alternate or even non-Mandarin pronunciations.
Take certain passages in 三家店 for example (listen for yourself in this video):

将身儿来至在大街口 (jiai)
尊一声过往宾朋听从头 (shen, pong, tin)
一不是响马并贼寇 (bin, ze)
二不是歹人把城偷 (chen)
杨林与我来争斗 (zhen)
因此上发配到登州 (den)
舍不得太爷的恩情厚 (qin)
实难舍街坊四邻与我的好朋友 (jiai, pong)
...

As you can see, the differences in pronunciation seem consistent, as if it is a formal part of Peking Opera instead of embellishments by the singers.
So is this true - there is a formal set of pronunciation rules for Peking Opera, analogous to a dialect? Where did this come from, was it based on one or many dialects, and if so which ones?
Beijing opera was not invented in the capital, but was largely imported from Anhui and other parts of the country, and then it evolved with further influences from other regions.

Certain pieces may therefore retain local accents, like Jiang-Huai (江淮官话) above, although the music itself makes it possible to just twist and distort the syllables.