If you're looking for somewhere to visit at the weekend that'll offer you plenty of photographic opportunities a castle is a perfect option, plus there are plenty of them scattered around the UK so you probably won't have to travel far to visit one. At this time of year castles tend to be quieter than during the summer months which means you'll be able to capture images with less tourists in them, too. Although, this can mean less activities are taking place if you plan on taking your kids along so it's always worth having a look online at what's on and when if you're looking for entertainment as well as a location for taking images in.
In most cases the best lens you can use is a wide-angle. The 10-20mm / 12-24mm super wide zoom is really useful to ensure the grander designed castles are fully captured in one frame. There will be some distortion if you're shooting at the wider setting, but this can be corrected in Photoshop or Lightroom. You could also use a longer lens to focus in on detailed carvings around the place, although, unlike cathedrals and churches, there's often less interest from a telephoto perspective. If, however, the castle is on a distant hill such as Criccieth Castle in North Wales or St Michael's Mount in Cornwall you may want to zoom in on the building and here a 70-210mm would be idea.
A tripod is not often allowed inside preserved castles, but is useful when you are allowed to use one. Not only will it help in dim lit interiors, but one with a built in spirit level will also ensure the horizon is level and the walls are perfectly upright. A light one is good especially if you're considering climbing to the top of the towers.
Also a compact camera bag is more practical option as the steps are often very narrow and you don't want to get stuck. A neutral density or grey graduated filter will be useful to darken a sky when photographing a scenic view of the castle.
Some stand majestic on hill tops others are surrounded by expanses of water filled moats, so a full view of a castle is sometimes hard to take. Pre-plan by looking at tourist brochures and postcards to find the best view point. Then use a map to work out how to get to that point. Google Maps, set to satellite view, can be invaluable. If you're shooting up a hill be prepared for the castle walls to slope inwards at the top. Some professionals may use a shift lens to correct the verticals, but at over £1000 they tend not to be high on most photographers' shopping lists. You can make corrections later in Photoshop, but be careful not to over do it as the result can look unnatural.
Walk around the inner walls of the castle looking for best vantage points. Shooting from a window in one of the towers can often give a better view of the rest of the castle. Climb to the top of a tower for a more dramatic viewpoint. Ruined castles are often harder to get good pictures as they're often badly damaged. In such cases look for interesting remnants and take close-up photos. Look for walls that have moss or lichen and shoot these for textures.
Windows and doorways are useful as frames for photos - shoot through one, but include it at the edges of the frame. Where possible make sure you're head on so the frame is parallel to the edge of the photo.
Steps and walls can make good abstract shots if you crop tight, but watch for light in interiors. There's often no artificial light so you're relying on shafts of light from windows, which tend to illuminate small patches of the interior. Our eyes adjust, but the CCD does not have the flexibility to cope with such exposure range. In such cases you can shoot several frames and merge them. This is known as the high dynamic range (HDR) technique.
In preserved castles such as Bolsover Castle in Nottinghamshire and Caernarfon in North Wales you will often find a calendar of events throughout the year aimed to draw crowds - one of these is re-enactments where people dress up in medieval costumes and act out scene of the past. This can provide excellent photo opportunities. A standard zoom is fine for most of this action. Increase the ISO if the conditions are dull and ensure you shoot at around 1/250sec to prevent subject movement. If a show is being put on aim to be at the "ring side" early so you get a good unobstructed spot.