If we may reprise Ibn Butlan (in Guy Lestrange's work):
"In the centre of the city is the church of Al Kusiyan. It was originally the palace of Kusiyan, the king, whose son, Futrus (St Peter), chief of the disciples, raised to life. It consists of a chapel (haikal), the length of which is 100 paces and the breadth is 80, and over it is a church (Kanisah) supported on columns, in which the judges take their seat to give judgement, also those sit here who teach Grammar and Logic."
He then goes on to mention the clepsydra outside the gate. As we noted in our comments on this clock the Chinese visitors place this clock outside the palace. However, was this really a palace? The clepsydrae had various uses in ancient times, but a key one was in timing speeches in law cases. In Roman times of course, the basilica was the main building for hearing these cases all over the Empire. Thus was this "palace" really just a basilica? We should note that basilicas were the most obvious buildings to convert to churches with the rise of Christianity and lend their historical name to many religious structures even now. Ibn Butlan also speaks of the "judges" making their judgements here. Many Romance languages (e.g. French, Spanish) still refer to their main courts as "palaces of justice".
So Al Kasiyan may have been the old basilica of justice put to new use. The church component was obviously the Church of Cassianus that Hugh Kennedy refers to in his work as the most important church in early Islamic Antioch.
What to make of this King Kusiyan? This king doesn't feature anywhere in the Seleucid royal annals and certainly is a bizarre reference considering the city was ruled by Romans when St Peter would have been doing his "reanimation" of the King's son. No-one has been able to offer an elaboration of who or what this means. It is probably garbled folklore.
Then, lo and behold, we have Stinespring's translation of the Vatican Codex. This it should be recalled, may date from the early Islamic period. The Codex says:
"And in the center of the city is the king's palace, in which are architectural ornaments, and columns of red, white and mottled marble; also in it are specimens of marvellous things, the descriptions of which cannot be portrayed. And it has seven high doors of iron, plated with pure gold; over each door is an idol-talisman, (so that) no cavalry horse of the (hostile) army can neigh and charge (against it)... And outside of it is the court of the judge and the magistrates."
This makes pretty clear that there was not only a court here but in fact a large political/judicial complex which probably was the Regia. The location though still remains a mystery. However, identifying the Cassianus Church's location would then give a site for the whole complex.
Then if we look at Kraeling's interesting writings on the Jewish community he relates an event slightly after 70 AD:
"A short time later, we are told, a tremendous fire did actually occur at Antioch, destroying the four-square market, the magistrates' quarters, the hall of records and the basilicas."
This makes clear that there was some sort of judicial quarter. Presumably rebuilding began shortly after. The new governor sent from Rome to take charge was called Caesennius Paetus, remarkably similar to Cassianus...
Also in Boucher we note a comment on officialdom that gives good reason why this complex would be a target for budding pyromaniacs: "The grammateus, or recorder, had under his control a grammatophylacium, where, among other records, were kept lists of State debtors, so that the building was exposed to danger from fire in the event of an outbreak by a reckless faction". This would be the aforementioned "hall of records".