There are now quite a lot of these: Moat, Lighthouse and Guardian offer straightforward immunity; Secret Chamber (retired), Beggar and Faithful Hound let the attack through but affect how it pans out.
It's easy to see mechanically how they differ, but which ones defend you best in practice? And which are the best to have in your deck when not being attacked? Would any of them be worth buying in a kingdom that had no attacks?
An alternative to defence is retaliation; instead of buying these, you could be buying attacks and trying to give as good as you get. Another alternative is repair: trashing the junk, draw-to-N, etc. Where's the balance there?
Do Hexes, with their unpredictable variety of effects, make repair harder and shift the balance in favour of offence/defence?
If you're going to buy these cards, when's the right time? Are they worth spending a $3 on before the first shuffle? Do you buy them in advance, or wait until your opponents have started picking up attacks? Guardian protects you the same shuffle you buy it rather than waiting for the next shuffle; how much of a benefit is that?