A.T. Robertson Commentary Acts 25:4

"Howbeit Festus answered, that Paul was kept in charge at Caesarea, and that he himself was about to depart [thither] shortly." — Acts 25:4 (ASV)

Howbeit (μεν ουν). No antithesis expressed, though Page considers δε in verse 6 to be one. They probably argued that it was easier for one man (Paul) to come to Jerusalem than for many to go down there. But Festus was clearly suspicious (verse 6) and was wholly within his rights to insist that they make their charges in Caesarea where he held court.

Was kept in charge (τηρεισθα). Present passive infinitive of τηρεω in indirect assertion. Hοτ with finite verb is more common after αποκρινομα, but the infinitive with the accusative of general reference is proper as here (Robertson, Grammar, p. 1036).

Shortly (εν ταχε). In quickness, in speed. Old and common usage, seen already in Lu 18:8; Acts 12:7; 22:18. Festus is clearly within his rights again since his stay in Caesarea had been so brief. He did go down in "eight or ten days" (verse 6). Luke did not consider the matter important enough to be precise.