King James Version
6 I have set watchmen upon thy walls,
which shall never hold their peace day nor night:
ye that make mention of the Lord,
keep not silence,
What are "watchmen"?
What does the Bible say?
Let's check these Bible versions:
On your walls, O Jerusalem, I have appointed and stationed watchmen (prophets),
Who will never keep silent day or night;
You who profess the Lord, take no rest for yourselves,
Common English Bible
6 Upon your walls, Jerusalem,
I have appointed sentinels.
Continually, all day and all night,
they won’t keep silent.
You who call on the Lord, don’t rest,
6 Jerusalem, I put guards on your walls.
They will not be silent.
They will keep praying day and night.
Guards, keep praying to the Lord.
Remind him of his promise.
Don’t ever stop praying.
Good News Translation
6 On your walls, Jerusalem, I have placed sentries;
They must never be silent day or night.
They must remind the Lord of his promises
And never let him forget them.
6-7 O Jerusalem, I have set intercessors[a] on your walls who shall cry to God all day and all night for the fulfillment of his promises. Take no rest, all you who pray, and give God no rest until he establishes Jerusalem and makes her respected and admired throughout the earth.
New Life Version
6 On your walls, O Jerusalem, I have put men to keep watch. All day and all night they will never be quiet. You who help the Lord remember, do not rest.
Orthodox Jewish Bible
6 I have set shomrim upon thy chomot, O Yerushalayim, which shall never hold their peace yom nor lailah; ye that remind Hashem, keep not silence,
6 Jerusalem, I have ordained keepers on thy walls, all day and all night without end they shall not be still. Ye that think on the Lord, be not still, (Jerusalem, I have ordained guards upon thy walls, and they shall not keep silent day or night, but shall always be ready to sound the alarm. Ye who think upon the Lord, be not silent,)
In SUMMARY, "watchmen" are:
men to keep watch