This is a problem. Mauls shows the route going south as:- Cross the harbour by the footbridge; Turn left and follow 'Quayside' all the way round. Climb the steps into the car park to reach the Old Grammar School. Then there is a gap in the route until the next waymark at junction of Queen Street and Farrants Way.
The IOM Survey 1:25,000 shows a different route:- From the footbridge turn RIGHT up Quayside to the Police Station. Turn left into Castle Street up to the square. Cross the square (The Parade) to Queen Street. Pick up the waymarked path at the junction with Farrants Way.
I've walked the area today, and found no waymarks between the footbridge and Farrants Way. The designated IOM Survey route is the clearest on the ground. However, given the lack of waymarks, I'm reluctant to change Mauls' route. In real life, Castletown is a natural start/finish or break point, so I don't think it matters anyway.
DECISION: I have added way 230335905 (Service Road /Parking Aisle) and added it to the nwn route to fill the gap.