Building a brick wall is a little complex. You will have to dig down and create footings even with a small wall otherwise it may fall down. However, as Daisy has suggested, you could buy or construct yourself, a seat around the tree like below. You can also buy metal ones.

They look cute too.

Or do you mean something like this?

Also, what are you trying to prevent?

Hi Juliet, what ever you do don't build up the soil round the trunk of the tree, the tree will eventually die. I made that mistake many many years ago.

Try some shade and drought tolerant plants if you want to grow owt underneath it.

The ring of bricks above give a fair idea of the look. irregular stone blocks might look more natural.

A narrow level trench (footings) will indeed be best to do it properly. Then some aggregate, concrete, and blobs of mortar (concrete with more sand added) between each block as you build the levels up. There will be countless vids online showing 'how-to'.

Hi Juliet. I would go for plants which like dry shade rather than any construction. https://www.rhs.org.uk/advice/profile?pid=430

Not much input from OP... 🤔