I am curious what a watch tower is? Is it a pele tower. A pele tower was usually a small three story tower ofen one would have to climb a ladder to the front door on the second level. Once inside you would pull the ladder up into the tower. They were made of stone and would be difficult to destroy by a few individuals.
There seems to be a big gap between watch tower and fortress. How bout a structure imbetween such as a smaller fort such as a Tower house which was a stone structure often in a L or Z shape. Many had angled turrets at the corners well above the ground. They had a compact footprint size and often in rugged terrain with difficult access. Often 4 to 5 stories high.
In a world where an Inn on a lonely road between to major settlements why wouldn't it fortify itself. Think of an Inn made of stone with a walled couryard with stable. While determined Bandits might break down the gate and the front door it would be difficult to destroy that Inn.
It seems to me even building a home out of stone would prevent it from being totaly destroyed by fire and axe. The thatched roof might burn and the furniture but the shell would still be there.
One final note. The first structure of a castle would be the keep, tower house then it would be added on to by walls, towers, gatehouse etc etc.
