Charles Ellicott Commentary Matthew 27:66

"So they went, and made the sepulchre sure, sealing the stone, the guard being with them." — Matthew 27:66 (ASV)

Sealing the stone—The opening of the tomb had already been closed by the stone that had been rolled to fill, or nearly fill, it. The sealing was probably accomplished by drawing one or more ropes across the stone and fastening each end to the rock with wax or some kind of cement.

And setting a watch—A better translation is with the guard. This means that the priests were not content to leave the work to the soldiers, but actually took part in it themselves.