Here "judgment seat" refers to Festus ruling over Paul's trial as judge. Alternate translation: "I sat upon the seat to act as judge" or "I sat down as judge" (See: [[rc://en/ta/man/jit/figs-metonymy]])
This can be stated in active form. Alternate translation: "I ordered the soldiers to bring Paul before me" (See: [[rc://en/ta/man/jit/figs-activepassive]])