1 Dwalin: Quick! Acrooss thu Bridge! {the party sets out across the narrow, precarious Bridge of Khazad-Doom}
2 Lambert: For all the amazing feats of construction engineering we've seen in Dwergenberg...
3 Lambert: For all the advanced technological skills and astoundingly beautiful architecture you dwarves have...
4 Lambert: Haven't you heard of railings??!!

We've seen the Bridge of Khazad-Dûm before in the fantasy theme, but this is the Bridge of Khazad-Doom. Entirely different thing.

We've also seen a reference to how bridges in fiction always seem to lack railings in the very next strip, which was in the space theme. This is just such a stupid trope that it's worth making the point again.

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2017-02-18 Rerun commentary: It looks like I've never explained how I made the visual effect of the bridge extending over a yawning chasm of darkness. In fact the LEGO plates are resting directly on a black clipboard folder. The clipboard doesn't come out that black in the original photos, so I adjusted the darkness in Photoshop to completely remove it from the images.

Many movie effects involving vast heights are filmed this way too, with the actors merely standing on a set built with a coloured floor just a few centimetres below them. The floor is then visually removed and replaced with an image of the Grand Canyon or whatever during post-production.

I was searching for an appropriate link to show how they do such special effects, but instead I ran across this Cracked article on 8 movie effects that were shot entirely in-camera, and not using computer generated effects, and it's so cool that I'll just link that instead.