Photo from the Sukkot Exhibit, curtesy of Neot Kedumim Park
הָעוֹשֶׂה סֻכָּתוֹ...בְּרֹאשׁ הָאִילָן אוֹ עַל גַּבֵּי גָמָל, כְּשֵׁרָה, וְאֵין עוֹלִין לָהּ בְּיוֹם טוֹב.
In the case of one who establishes his sukka at the top of a tree or atop a camel, the sukka is fit, but one may not ascend and enter it on the first Festival day because the Sages prohibit climbing or using trees or animals on the Festival.