unasked additions...
1) heavy armor is better armor (protection from normal enemy attacks). Light armors have bonuses. A couple of heavy pieces also have a bonus, but its unusual, I can only think of one and its a repeat of most cloaks. I could be forgetting something. I find a mix of heavy and light most useful late game, specifically an anti- pierce piece is really good.

3) yes, raise walls are one of the best defenses in the game. another is the reverse, digging a moat. Defenses that do damage, almost all enemy are aware of these and avoid them, making them more effective than the actual damage done. Spiked walls may be an exception, or something is buggy about how they detect these, or they are both attracting and repelling so they get confused, I really don't know, but those are not quite as good as other damaging defenses.

4) you should check it out. It could be anything from normal boat noise (boats are incredibly noisy in game) to enemy slapping each other (night spawn enemy include undead who fight all other enemy and cross biome monsters that also hit each other somewhat) to something gnawing on something you built. Best to satisfy yourself whether its anything important.