… but I just had a look at the gate during construction:
http://guildedage.net/comic/annotated-22-4/
and that looks solid to me. There’s retentions in all directions alright! And not just that, but the base of the gate extends far enough that even during construction, the lower blocks would probably not tip over while the next remaining blocks are stacked on them. It may or may not be physically accurate, but this is good enough for me.

Impressive level of planning, John! That is way beyond what I expected!
(but that doesn’t preclude some more internal geometry which can prevent the stones from sliding out

Each stone has its own weight pulling it down, and whatever forces its neighbours are exerting on it. Since there’s an angle between the stone’s lateral faces, the pressure from its neighbours is pushing it out of the circle. On the upper half, that is counteracted by the stones’ own weight. On the lower half, the weight of each stone is also pushing it outwards, so it’s not going to work.

As John correctly says, of course: If the faces are not plain or are prevented in some other way (braces in the back?) from sliding off each other, it could still be made to work, although it’s not an efficient structure.

I’m going to assume that, like the torches that are being shown in silhouette, there’s a stylistic reduction of detail for objects that aren’t the scene’s intended focus, and that’s a contributing factor in why we aren’t seeing them.