Is it recommended that we purchase tickets ahead of time for attractions in Edinburgh in September?

The castle, probably yes. You could go to the website and look at the days you’re interested in - it actually gives the availability in numbers for each time on every day, so you can see how close to selling it out it is.

The main art and history museums in Edinburgh - the National Galleries, the National Museum of Scotland, the Portrait Gallery - are all free admission so you just turn up without booking.

I believe pre-booking is recommended for the Real Mary King’s Close.

Definitely the castle. My daughter was there last year and didn't buy in advance and it was sold out.

If you want a tour of the Scottish Parliament Building definitely do that now as they book up early (may be too late if going this September). We really like Holyroodhouse Palace across the street but you won’t need to get advanced tickets for that.

In late September 2022 timed entry was only needed for the castle. Both Holyrood Palace and HMY Britannia had short lines to get in. If you intend to visit Rosslyn Chapel, it too only had a short line. When we visited its Internet was down so credit cards could not be processed. Fortunately we had cash; many others there didn’t.

We really like Holyroodhouse Palace across the street but you won’t need to get advanced tickets for that.

Yes, but keep in mind that sometimes the palace is not open to visitors for periods of time, so you might want to check ahead to make sure that the open dates correspond with when you are there. Although I just checked and it looks like September is pretty much open (I'm assuming you're talking about 2024).