There are many ways to take fortifications with poles, but these are my favourites:
Make a ladder: Cut one pole in several parts and tie it between to full-length poles. Can be repeated to make several ladders. Use the ladders to climb onto the walls.
Undermine the walls: Dig tunnels beneath the walls and support them with poles. Once you have dug enough, set the poles on fire and evacuate the diggers. The tunnels will collapse without the support of the poles, and hopefully so will the walls that stand above.
Starvation: Build a palisade of your poles around the besieged fortification to make it harder for the enemy to sally out and break the siege. With nowhere to get supplies from, the besieged will eventually surrender or starve to death. If you have many poles, you should also build a palisade defending your camp against forces trying to relieve the siege from the outside.