This was my second favorite game of the competition. The amount of content (for a demo, even!) was really impressive, and the game is polished and fun. I think you did a great job with this and I intend to check out the full version.
I encountered one technical issue playing the game in Mesen where a wildcat enemy in the beach area (on the map, the sprite dot for this screen is at $70,$97) spawned incorrectly, with incorrect attributes (vertically flipped when still and wrong palette) and incorrect properties (would not collide with the player or projectiles). When it spawned, the music also broke, only playing on the triangle channel. I rewound and tried again and it did not break the second time, so I'm not sure what triggered the problem.
I have one major complaint about the game, which is that the balance for enemy health and weapon damage seems really off. I got pretty far before finding my first weapon damage upgrade, and by that point, enemies had turned into damage sponges. I had to resort to turbo because it was completely unreasonable to try to kill them with actual button presses, and even with turbo, some still took way too long, especially the bosses. I'm not sure if I just picked the wrong path every time there was a fork and thus missed out on all these critical upgrades the first time through, but without any damage upgrades, the combat becomes a frustrating, time-consuming chore. I did eventually decide I probably missed some kind of combat upgrade and should go back, but the world is so large and wide, so with how much distance I'd have to cover to return to the earlier areas, it seemed better to just keep marching forward rather than turn around and explore more in the hopes of maybe finding useful items I didn't actually know existed.
As such, I wish the game did something like any of the following:
• There were perhaps damage upgrades on the main path
• There were something to gate forward progress until some useful combat upgrades are found
• Damage were reworked to make later enemies immune to the base gun to help the player understand he's missing something
• There were earlier access to a fast travel mechanism so going back to explore more isn't so time-consuming early on
Any of these probably would have solved this damage problem for me, and it's easily the biggest issue I saw and one that might prevent me from recommending this game to someone who isn't willing to use turbo.
Despite that, I still really enjoyed Isostasy. I'm really impressed by the size of the world and how well you used the repeated screens, as it generally feels more unique everywhere than Metroid. The style and graphical touches, like sprites for an extra 'background' layer or the changes to the player sprite in different environments, are really nice. The map is also extremely handy and works well. Thanks so much for submitting this great game!