I was thinking of ways to keep the HUD simplistic but also powerful since it's being optimized for iOS with small screens. I came up with this design for creating units. You click on a building and it'll become selected. A popup shows up asking if you want to check your units.
If you click yes then the HUD at the bottom will scroll up and reveal more options for creating units. This way the units will be specific for each building. For example if you selected a ranch you would be able to create more horses for the cavalry, etc.