Great post. I'd never heard of Bruegel's Babel before, but a quick google yielded this pic:
http://images-jp.amazon.com/images/P/3791319418.01.LZZZZZZZ.jpg
Actually, I was influenced more by the LOTR films, specifically Helm's Deep (the valley and the stone road up to the the main gate). Actually, the road undulates quite nicely, a feature not readily apparent from the high vantage point in that shot. The tiered construction of the fortress was a nod to Minas Tirith, whilst the huge beacons were inspired by, well, the beacons. Only just visible on the grassy court above the main entrance, partly obscured by the spiky turret is The Brown Tree of Kinndor >:)
Totally agree with the points you made regarding the textures. I'll definately work on getting some more variation in there. My biggest problem with the Q3 textures are that they aren't the best tiling textures in the world. Take the yellow road texture for example; it's scaled 2x in both axes on the road, and the tiling is still pretty bad; i'll likely replace that. Damn good point with the water as well - that'll be replaced.
The rock texture will stay as it is; in fact I think it looks even better from the ground, and it really blends in nicely with the mountains in the skybox, which I think is perfect as it is :)
Of course, I'd love to see some of your recent skies, but I think I've settled on the sky for this map.