3D on the GBA always seemed woefully unexplored. I mean this port of THPS2 was a launch title:
As far as I've ever been able to tell, there are no polygons.
- Player and enemy ships + missiles etc are flat 2D scaled sprites - Some backgrounds are pure FMV (I think some warping is applied as the player ship moves, to give a more 3D feel) - Some backgrounds are "SNES Mode 7" style ceilings and floors
Probably the definitive example of "faking 3D without using any polygons" IMO. Even more impressive than Sega's super-scaler games like Space Harrier, Galaxy Force, etc.
I'm seeing a pre-rendered background loop with a little skewing applied as the player moves from one side to the other, with a lot of scaled sprites on top of it. Nicely done.
The big tell on the background loop for me is that the repeating background stuff never changes during a level.
Not sure about Iridion 2 but some levels in Iridion 3D are not purely prerendered background loops - this level is a Mode7ish flat "floor" (the ocean) with a perspective-warping sky BG
It's not super advanced technically, maybe, but it's very well done IMO