When sending a Zergling, Marine, or Zealot to a Xel’Naga Watchtower, be sure to unbind that unit from your army hotkey!
The easiest way to do this is:
- Have your whole army selected (and hotkeyed as #)
- A-click to the Watchtower
- Shift+left click on the unit you want to leave at the Watchtower
- Re-hotkey your army by hitting Ctrl+#
- Move your army wherever else you want
Remember that you can shift+left click on the unit meshes at the bottom of the screen if you do not have the mouse accuracy to quickly left click on a specific unit in the middle of your army.