It may get very hard for your soldiers to keep an eye at every part of your land. So you can help them by building Ballistatowers.
With their help your sight will increase and you may be warned against enemies earlier. The ballistatowers can help you defending your town. They are also a more efficient version of the watchtower , because they can deal damage to your enemies.
To provide your watchtower with firearms, obtain the knowledge of gear wheels so you can then upgrade them to ballista towers.