Frothers Unite-UK has a gallery of gothic and fantasy terrain pieces. There are a lot of commercial products here that I’ve not seen before.
Chris Brantley has a page with advice on scratch-building wargames terrain. It’s aimed at newbies in general, and DBA players in particular.
Manorhouse Workshop has a set of three tutorials on making wargames terrain from high density polystyrene (insulation board).
Battle Brush Studios has a tutorial on making a dark ages / medieval village.
The 1000 Foot General blog has a good tutorial on building rocky desert hills that are suitable for a wide variety of settings.