Acts 22:30
Before the Sanhedrin
30The next day, since the commander wanted to find out exactly why Paul was being accused by the Jews, he released him and ordered the chief priests and all the Sanhedrin to assemble. Then he brought Paul and had him stand before them.
Paul was a Roman citizen...Roman citizens had certain legal rights...They could not just throw him in jail and throw away the key like they could non-Roman citizens...
The Roman government let the Sanhedrin hold certain legal rights and courts of sorts to judge and jury in many cases...Paul's case was a religious case very much suited to be brought to the Sanhedrin...Paul being a Roman citizen, the Roman guards and centurions would closely watch this case before the Sanhedrin...