What is a Context Menu and what is it for?
A Context Menu will appear if you mouse-over a building and left-click once. You can open your building by double-clicking.
These menus will change based on the building, however, every building below level 10 will give you the option to Instant Upgrade, Upgrade, and view the Details of the building!
Below is an example of the Context Menu for a Level 1 Barracks: