fortification bonus within walls or fortress

[Sieve Too]

All bonuses are multiplicative, none are added.

From tests I've run, you only get one fortify type bonus: The highest of X1.5 for unit fortifying, X2 for a fortress or X3 for city walls. There is no advantage to fortifying a unit in a fortress.

[Blaupanzer]

A fortified unit in a fortress or city uses that bonus against bombers and howitzers (which ignore the walls). Always fortify. It's a good habit with no downside.