Fortify units

Hi all! Here's an idea, I am curious for your opinions. So, what if...

..we could fortify units, or at least land units. It means that, if we fortify a unit it's defenses will be +2 higher, but the unit can not moved for 2 turns by it's owner. It will be "locked" for it's position, wether it's in a city or just anywhere on the map. So, opinions?