Ye have a watch ( ἐ ìχετε κουστωδι ìαν, take a guard). Pilate answers briefly and haughtily, "Well, I give permission; do as you like; take a body of soldiers as a guard, and go your way." This last verb is imperative, so the former is most probably imperative also.
If taken as indicative, the question arises—What guard had they? This is difficult to answer, unless, as Alford supposes, it may refer to some detachment placed at their disposal during the feast. But of this we know nothing historically.
Make it as sure ( ἀσφαλι ìσασθε, secure it for yourselves) as ye can; literally, as ye know how. Take any precaution you think fit to employ.