You can make an easy watering system for use with potting soil.
Take a 1" pvc pipe the same length as the planting tube, using a 1/16" drill bit drill holes on opposite sides of the pipe about 1 foot apart starting about 2 inches from one end. Then put a pipe cap on that end. On the other end attach a 1" pressure S x FPT Female Adapter.
Cut the bottom off a 2 liter soda bottle & screw the bottle onto the adapter. Place the watering pipe inside the planting tube then fill in with soil. Then when you need to water the tower, just fill the soda bottle with water & it will drain into the tower, watering at 1 foot intervals all the way down.
You can add more water as the bottle empties if needed, plus when it rains you can put a large funnel in the bottle to catch rain to water.