Sealing the stone. — The opening of the tomb had been already closed by the stone which had been rolled so as to fill, or nearly fill, it. The sealing was probably effected by drawing one or more ropes across the stone and fastening either end to the rock with wax or cement of some kind.
And setting a watch. — Better, with the guard. What is meant is that the priests were not content to leave the work to the soldiers, but actually took part in it themselves.