Carephilly Castle at Twilight

Although this image only shows a small bit of Carephilly Castle, it's the reason I took the shot and hence I've titled it thus.
For a brief moment, the sky developed this lovely purple hue which soon subsided to the usual tungsten light stain due to the local street lights.
I only managed to get a few shots of this purple/twilight style hue and this is the better of the two.
A combination of a ND grad filter and tripod allowed a long exposure with an even exposure acros the frame. I wanted a square format when I took the image, so I took it as a portrait orientation and composed the top to how I wanted the image to look. I cropped later in photoshop.
Exposure accuracy is a real headache on long exposures because the light levels are constantly changing, even during exposure. I find that evelative metering is best, but I usually need to compensate after a few gimp shots first!