Howbeit (men oun). No antithesis expressed, though Page considers de in Act 25:6to 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 (Act 25:6) and was wholly within his rights to insist that they make their charges in Caesarea where he held court.
Was kept in charge (tēreisthai). Present passive infinitive of tēreō in indirect assertion. Hoti with finite verb is more common after apokrinomai, but the infinitive with the accusative of general reference is proper as here (Robertson, Grammar, p. 1036).
Shortly (en tachei). In quickness, in speed. Old and common usage, seen already in Luk 18:8; Act 12:7; Act 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” (Act 25:6). Luke did not consider the matter important enough to be precise.