Many top attractions in Edinburgh are free, but adults (age 16 and up) pay £14 ($23 USD) for admission to Edinburgh Castle. Yes, the price is steep. But when you consider it's possible to spend half a day here, the cost is a bit more tolerable. Remember that admission prices sometimes vary during special events, and operating hours change with the seasons.
If your travel plans are firm, it makes sense to pay for Edinburgh Castle tickets online and print them prior to arrival. In summer, this almost certainly means skipping some long lines.
Guides are available on the grounds to give you a short tour. If you have mobility problems, be aware that there is some light climbing required as you explore.
You can see the Scottish Crown Jewels here, observe one of Britain's only cemeteries for soldiers' dogs and see the cannon from which, at 1 p.m. each day, a single shot is fired. Our guide explained that the Scottish are frugal, and chose not to fire it at noon each day so they would only have to pay for one round each day instead of 12.
I think most budget travelers can respect that philosophy.
