Why is the North’s western shore so poorly defended against Ironborne? Seagard has its belltower and the Reach has watchtowers and the Shield Islands, but none such structures are ever mentioned for the North.

A couple reasons:

1. The North’s population is more interior than coastal, so the shore is less important than it is in the Westerlands or the Riverlands or the Reach.
2. The North had better defenses at various times – “Balon V Greyjoy,
   called Coldwind, destroyed the feeble fleets of the King in the North…We have had no strength at sea for hundreds of years, since Brandon the Burner put the torch to his father’s ships” – but have neglected them in recent centuries.
3. Even without those defenses, the Ironborn don’t do well trying to hold the North, in part because “ironborn lack the discipline to stand a charge of armored horse.”