The Gemara in Makos 12b explains that according to Rebbi Yehudah, if the trunk of a tree is in an Ir Miklat and its branches extend out of the Ir Miklat, a Go'el Ha'dam may use the trunk as a step to climb up the tree and kill the Rotze'ach - who is sitting on a branch of the tree.
But I thought there was no entry for Go'alei ha'Dam into the Ir Miklat - how did he get to the trunk inside of the city in the first place!
Rabbi Eli Liff, Har Nof, Yerushalayim
The Mishnah in Makos 11b tells us that even the Techum, or 2000 Amah vicinity, of the Ir Miklat is Kolet. Although the Arei Miklat certainly had walls, and would have guards preventing entry to Go'alei ha'Dam, there could not have been a wall around its Techum.
Thus, the Gemara is referring to a tree at the edge of the Techum of the Ir Miklat, to which the Go'el would have easy access.
Kollel Iyun Hadaf