A building with battlements is a multiple part building ...
Units on battlements are not Fearless like other embarked units.
Unlike the interior of buildings, to which we grant a Transport Capacity, the battlements of a building can hold as many models of a single unit as will fit ...
If there are external ladders, or similar Access Points that only provide access to the battlements, a unit can embark directly to the battlements ...
If the interior and battlements are both unoccupied, a unit can embark to either the interior or the battlements.
If the interior is occupied by a friendly unit and the battlements are empty, the embarking unit ...
If the battlements are occupied by friendly unit and its interior is empty, the embarking unit ...
If an enemy unit is occupying the interior, you cannot move through them to get to the battlements. Unless there are external Access Points that provide direct access to the battlements, the unit cannot embark to the battlements.