If you're referring to that hour just after sunset, when the sky takes on that deep blue hue, but not full-on nighttime, then you really don't need a very long shutter speed. I'd tend to agree with Aperture priority, probably with the kit lens but mostly dependent on how wide you want to be. Camera on tripod - preview your white balance ahead of time (the great thing about live view on your A33 - you can see what white balance gives the color tone you want right on the screen or EVF!). Use a middling aperture initially - the good old rule of F8, and set ISO to the lowest setting, then let the camera decide the shutter length needed to get a good exposure. With the A33, I'd recommend using center-weighted metering mode rather than multipoint or wide. If the castle overexposes too much, set to -.3 or -.7 EV, and try again.
You can also try a shot or two with your 35mm F1.8 set wide open, ISO to around 800 to 1600, and shoot handheld - use the multistacking ISO mode for this, so you can stack the 6 consecutive frames to reduce noise allowing the higher ISO...you can often get a nice shot even without a tripod at that hour, and when properly exposed your camera should have no problems delivering a clean ISO800. Here's one I snapped handheld with my Sigma 30mm F1.4 lens at F1.7, ISO800, at around a quarter to 9pm:
For that special magic blue sky hour, you often don't want to run the shutter length too long - that blue glow is usually easy to catch with a slightly higher ISO and a shortish shutter speed. Longer shutter speeds will sometimes overexpose the sky and lose that blue hue a bit. I'd stick from handheld to no more than around 4 seconds or so for blue hour type shots...a few blue sky night examples:
This was 4 seconds:
This only 2 seconds:
You also might want to try a few HDR exposures - night HDR using the in-camera HDR mode is extremely effective in allowing you to control specular highlights and streetlights in shots, while still nicely exposing shadows.