en_tn_condensed/mat/27/65.md

a guard

This consisted of four to sixteen Roman soldiers.

sealing the stone

Possible meanings are 1) they put a cord around the stone and attached it to the rock wall on either side of the entrance to the tomb or 2) they put seals between the stone and the wall.

placing the guard

"telling the soldiers to stand where they could keep people from tampering with the tomb"