Art De La Guerre

[Nostrebor]

The rules state that any unit fighting an enemy across a fortification can disengage.

If someone in front the fortification disengages and a new unit replaces it. I would think this is a new charge (same as if a a unit in the open that were allowed to disengage did so and a new unit charged to replace it).

If someone behind the fortification (i.e. defending it) disengages and is replaced is the replacing unit considered to be charging? Does it get the benefit of "defending a fortification" on the turn it moved or only on subsequent turns.

The rules do state that impact and furious charge are canceled when fighting across fortifications. Perhaps this, in effect, cancels all effects of charging (in which case the question is moot).