Do my units get a defensive bonus when fortified? Like in Civ 5? How much is the bonus? The tooltip doesn't say and I could not find an entry in the civilopedia.
Yes, fortified units get a defensive bonus. The bonus starts after one full turn of fortification (meaning that if you spend 1 movement point and then fortify you won't get it until the next turn). You can confirm you have the fortification bonus by seeing if the unit icon has changed from a circle to a shield. If you remain fortifies for 2 turns the fortification bonus increases.
I don't have the game open now, but I believe I remember from the tutorial that the bonus you get is +25% defensive strength per turn for up to 2 turns.
Additionally fortified units will heal themselves if injured, and will not come up in the units needing orders list unless there is an enemy nearby.