Right now, the placement of the City and the Castles seems rather random. Usally when generating a map the city (which is called trading “hub”) and the castle are on end points of the road net. Only the watchtowers seem to be in good places.
The city (or cities in the future) should be in the center of civilisation. There should be a high probability for the city to spawn next to a river – those were important delivery routes in medieval times. The roads from the city lead to the “important” villages. Those are the roads that should be protected by watchtowers. The important villages should be near a “important” location – near a bridge, in open fields with great farmland, near some mountains for quarries/mines. From those villages unguarded pathways lead to the “unimportant” villages.
The castle should be near the city – for the protection of the leading nobles – or near an important strategic point, like a mountain pass or a bridge.
This will work with your “great evil invasion” plan – the outlying unprotected villages will fall first, then the more important villages and the city and castle will be under siege at last. Right now the great evil could start laying waste to the city from the start.
If you have some settlements near one river you could even generate “protect the barge” contracts, which could lead to pirate action…