Facing an imminent attack requires a careful balance of faith and practical effort. The response to the looming danger involved a dual strategy of turning to God in prayer while simultaneously organizing a physical defense by posting guards.
The primary focus of these guards is understood in two distinct ways. One perspective suggests that the guards were positioned directly over the builders and the residents of Jerusalem to physically shield them [רש״י, מצודת דוד, ביאור שטיינזלץ]. In this view, the security detail was established specifically for the sake of the workers, ensuring their immediate safety as they labored [אבן עזרא].
Conversely, another approach explains that the watch was oriented outward toward the enemies themselves. Scouts and troops were deployed to monitor enemy movements and provide an early warning system. This ensured that if the hostile forces advanced toward the city, the people would be fully prepared to fight back [רלב״ג, מלבי״ם].
Ultimately, this entire state of high alert was a direct response to the constant intimidation from hostile neighbors, led by Sanballat and his associates [רש״י, מצודת דוד].