By "like castles" I dont mean high stone ones, but say a couple of sandbags that are higher than ground level with alternating gaps. Every depiction i see has either a bunch of sandbags level to each other, or just the natural flat earth. Wouldnt having some sandbags jutting out offer more protection, especially from angles? (Most importantly) What was the rationale behind no longer using the method of olde? I suppose this also technically applies to modern warfare as well, but i guess same-same.
They did. I mean maybe not universally but it was pretty common, certainly in the British trenches. A wartime cartoon from Punch shows a British officer home on leave and watching a production of Hamlet and asking the gravediggers "AINT YER GOING TER 'AVE NO PARAPET?"
Literally every diagram of trenches Google images throws up shows a sandbag and/or earth parapet for example this, this or this.
You can see them in photos like this, this and this.
Now I think what you're imagining is crenellations, with gaps to look and fire through - and the simple answer is that instead of that they had loopholes within the parapet, just take out a sandbag or two and you have a hole you can look and poke a rifle through. Often they were reinforced with wood to stop the other sandbags collapsing - you can see this clearly in this photo of a reconstructed German trench here, and in use in this re-enactment. Sometimes they would be covered when not in use either by simply stuffing them with fabric or using loophole plates like this Turkish one held in the Imperial War Museum in London.
And yes, the good old sandbag parapet is still going strong, as seen in this picture of a Ukrainian soldier in March 2018.
