I'm confused - when do restrictions like "there must be Invaders" for Build and "there must be a source of Explorers" for Explore apply?
I'm confused - when do restrictions like "there must be Invaders" for Build and "there must be a source of Explorers" for Explore apply?
Those apply when resolving Invader Cards - specifically, when making a list of what lands match the Invader Card (JE rulebook p. 27)
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)
