If an Event tells you to perform a single Build action - eg, "On Each Board: Build in a land with Dahan" - you don't check whether that land has Invaders.
But if it tells you to resolve an entire Build Step / Build Card for a particular terrain, it works just like it would if you had a physical card - you're making a list of which lands match, so the normal Build/Explore restrictions apply. The Events "Fortification" and "New Cash Crops Take Hold" work this way (and do not usually build in empty lands), though it's not how they're phrased.
(See the Errata in the JE rulebook p. 27)
Tags: Build, Explore, Ravage, Events, 2 more...