If you're pretty much resigned to auto mode, you will at least need to learn about ISO levels in order to get the most out of the camera.
ISO is a way of measuring how sensitive film is, or in the digital world, how sensitive the sensor is. By sensitive, I mean how sensitive to light it is.
Most cameras start at either ISO 100 or 200, then go up exponentially - 200, 400, 800, 1600, and sometimes 3200 or even 6400. Each one is twice as sensitive as the one before. For example, assuming that everything else is the same, a picture that must be taken at a shutter speed of 1/30th of a second at ISO 200 can be taken at a shutter speed of 1/60th of a second at ISO 400 or 1/120th at ISO 800. The faster the shutter, the less blurring you'll get.
Out of the box, I'm pretty sure that your camera will only use ISO 100-400, which is fine for daytime shots when there's plenty of light. But for nighttime shots, you'll want to manually set the ISO higher.
Now, why wouldn't you just set your camera to the maximum ISO and leave it there? Because at the higher levels, the sensor will also have "noise", with you see in the final photo as blotchiness, speckles of color, loss of sharpness, etc. By and large, you want to use the lowest ISO possible, but you don't always have a choice. Digital SLRs generally can deliver nearly perfect quality up to ISO 400, ISO 800 has slight amounts of noise, and ISO 1600 has slightly more but still not very much.
To sum up - ISO and shutter speed are like two sides of a teeter-totter - as ISO goes up, shutter speeds become quicker, and when ISO goes down, shutter speeds become slower. Your current night photos of the castle are blurry from camera shake because the shutter speed is too long, so a higher ISO will allow you to use a faster shutter speed, minimizing camera shake and leading to a sharper photo.
If you "get" that, congrats, you're well on your way to the next step... which is aperture, which also teeter-totters with the ISO and shutter speed.
Remember, with a DSLR, the camera is capable of delivering darn near any photo you want - if you want the best it can deliver, you will need to speak its language, at least a little. Throwing expensive lenses at it helps too, but a good understanding of the basics is worth more than a great lens, IMHO.