Watchtowers or outposts for trade routes and borders is the thing that we will see in bannerlord? If not, that would be both historical-wise(Roman "watchtowers" or its superior "Castra") and gameplay-wise(before you arrive on that location, your small garrison make delay the enemy activity and save time for reinforcements) good. You can put some garrison there to secure and watch the area. This also adds small-sized sieges or battles and some visual variety(wooden watchtowers or wooden forts).
"buildable watchtowers or at the tier 2: (fort) idea" if a vassal or king build a watchtower in the near of the enemy border; this may cause suspect and worsen their relationship with the other kingdom at the same border, so it should be both buildable by its owners and destructible by its owners and enemies. In any case, no harm in build them across trade routes.
"buildable watchtowers or at the tier 2: (fort) idea" if a vassal or king build a watchtower in the near of the enemy border; this may cause suspect and worsen their relationship with the other kingdom at the same border, so it should be both buildable by its owners and destructible by its owners and enemies. In any case, no harm in build them across trade routes.