Crichton Castle is managed, maintained and run by Historic Environment Scotland. It is located at Pathhead, near Dalkeith in Midlothian - not that far away from Edinburgh. There is a fee to enter, or its free if you are a member of Historic Environment Scotland, or an associated organisation (eg, English Heritage which we have membership to).
Although it is not that far from Edinburgh, and also near Dalkeith, it was quite a drive to get to it - as it is literally in the middle of nowhere - and you have to go down single track roads to get to it - but all were easily driveable. It is then situated on the side of a hill, although it is a fairly straight and flat walk over to it - and once there, there is a small staffed Historic Environment Hut for entry, with info and the usual gifts, guide books and the like. 
Although a ruin, you get pretty good access, and it is maintained, so its fairly safe. It was built in the 1300s and has a fair bit of history surrounding it, of which you can read more on the Historic Environment Scotland webpage for it. It was very quiet when we were there, with only one other group of visitors around - so we pretty much had free reign of the place. As per usual with these sites - it was pretty good for kids - lots of space, information boards and you could explore the castle and the associated outbuilding.
The most striking feature was a diamond shaping to some of the inner walls.
I've never seen anything like it before in a castle in Scotland, and on first seeing it, I thought it was a more recent addition, say as part of strengthening works. But as per the info below, it was actually carved in place in around the 1580s - which is quite incredible:
Some of the other construction was quite impressive, and in the following picture you can see quite large vaulted rooms - which must have been no mean feat when constructed all those years ago.
The large stables next to it are also quite significant. From a distance they look like a large chapel standing next to the Castle, especially noting the buttressing on the side walls.
The castle is quite tall at around 3-4 stories high, and would have made quite an imposing structure all those years ago. The reconstruction below helps us to understand better what it must have been like:
As with many castles in the area - it has royal connections, and allegedly Mary Queen of Scots had made a visit. In Scotland she is the one Scottish Royal figure everyone wants to know about, and if she ever visited a castle or other building - it is featured heavily in their literature, as all these years later, she is still a big draw for visitors and tourists.
Here's another picture of that diamond carving - it really is quite something else:
In summary:
- Crichton Castle is well worth the visit; the journey, the location, the short walk to it and the castle itself are all worthwhile - we'd happily go again,
- Kids will enjoy it with lots of things to explore and see and do,
- The diamond carving feature makes it quite distinct, and if architectural features are your thing - then its worth it just for that.
Links:
If you want to support important buildings, history and heritage in Scotland and the UK, then please consider the following:
For a reasonable annual membership price, both provide access to numerous historic properties across Scotland which are great for days out. Once you've been a member for more than a year you also got free access to other properties across England, Wales and Northern Ireland (including half price entry during your first year) - so its well worth the investment, plus its all for a good cause.
