From Halopedia, the Halo wiki
The Watchtower is an automated structure used by the UNSC to defend Firebases by detecting cloaked enemies. Additional Watchtowers extends the cloak detection range, and each Watchtower occupies a turret slot. The multi-modal stealth and jamming systems employed by the Covenant and Banished are a significant strategic challenge for the UNSC. Though improvements have been made, there is still a significant capability gap in detection range versus cloaking capabilities. Nevertheless, with microdrone clouds and hyper scanner technology acquired from former Covenant sources, UNSC firebases can deploy a short-range sensor network that will reliably reveal enemies otherwise shrouded by cloaking generators and active camouflage. Once detected, their location can be shared to all friendly units for targeting.
Halo Wars 2
During Campaign, the Banished cannot attack the Watchtower, but on Skirmish, it can be attacked. The watchtower itself allows player to monitor enemy movement and detects any possible threat coming to the Player. The Watchtower is introduced in From the Deep where Alice-130 was stationed on defensive mission on beachhead. The Watchtower can be deployed by Anders in the Blitz gamemode.
List of appearances