。
The court convicted him at the trial.
Literal
Court [at-では] him [object-を] guilty-[quotative-と] judged
判決する (the verb form) is used here instead of the more periphrastic 判決を下す. 有罪と判決する ('to rule as guilty') collapses the object and the decision into one compact verb phrase. The initial 法廷では frames the court as the venue.