As a Rank 7 District, the Ranged Towers will not be seen until at least one defense-minded district reaches this high level of development. Your settlement has been running for a long time and probably has access to everything needed to build this district. This is a solid addition to your Department of Defense.
Both the Main and Exclusive buildings of the Ranged Towers require some preliminary research. the Defensive Tower requires you to have some mastry of Masonry in order to build a Brick building that high. By the time your settllement reaches Rank 7, it probably has already researched both Mathematics and Masonry enough to start this buildings. The exclusive Ballista Towers require Engineering knowledge, which relies on the base-research of Science.
Action - Practice Archery
Living in a settlement that contains a Ranged Towers allows players to partake in the district action. By visiting this district, residents can Practice Archery, increasing the effective Might of all troops in their current Brigade/Regiment. This bonus lasts until the end of the next Harvest cycle.
Main - Defensive Tower
Building your Defensive Tower requires Bricks and Leather but by this point your sovereign probably has access to both of these items or the settlement wouldn't be so big. Setting some of that aside for construction, maintenance and upgrades to the Defensive Tower should be fairly simple. This building trains every kind of unit currently available to your settlement, making it a powerful addition to the Department of Defense. Each level of Defense Tower supports two completely free Archer units who defend the Ranged Towers district.
Exclusive - Ballista Tower
The Ballista Towers can only be built in the Defense Towers district and their units only defend this district. Every Ballista tower is home to 2 completely free Ballista units per level that defend the Defense Towers from any invasion force. Ballista Tower require Bricks and Canvas to construct, keep up and improve.
Linked - Fletcher
Fletchers are your primary source of Ranged Weapons. They can be built in any district but are most efficient in the Ranged Towers. They need Leather and Pet Birds to construct and harvest feathers from and offer no positive or negative Happiness effects to district or settlement.
Minister - Magister Inflectum
In charge of hitting stuff at a distance is the Magister Inflectum. They have a seat on the Department of Defense and are one of the few Ministers with the inherent power to train any kind of troop that's been researched. This leaves them and their Ministry of Ranged Warfare in a relatively powerful position among their fellow Ministers of Defense.