I was very fond of a large holly tree in our garden. Its prickles were very aggressive but there were lots of branches, so it was easy to climb and below it there was lots of dense undergrowth for hiding from and spying on people. Not that anyone but the gardener went by. His name was Mr Moon and my sister and I locked him in the outside lavatory once. We thought it was very funny, but got told off.
Another tree I liked was a very ancient yew tree which overlooked the tennis court. That was my summer climbing tree. I liked that because it had a huge trunk and I could get up into it and shout abuse at older siblings playing tennis and chuck tennis and dog balls and the like at them.
Many thanks to Lorina Pilgrim, investor in people and tomboy