Originally Posted by
Bloodly
Many have felt this. As such, when it was discovered that it is possible to protect your merchants, many jumped on it. The secret in the default game is to either stack them with a troop placed on a trade resource, or with a carefully placed fort with a troop inside it to keep it from vanishing. This also has the benefit of letting multiple traders trade the same thing and enemy traders can't mess with troops or buildings. The risk is with troops without a general or a general with low loyalty defecting and thus losing the merchants stacked on them.
In mods, whether this is possible varies. Here, you can't place forts on trade resources.