Thanks for your feedback! The disconnected platform and not using platforms in the spiral staircases, both at level one, are great points to which I can't recall any reasoning behind the current design, so we'll change those.
Regarding the usage of spikes, I see now how the first level can be quite punishing for new players, so we'll redesign that section accordingly. But a point we forgot to mention on this week's devlogs, is that levels have many invisible "checkpoints" at "solid" platforms. Therefore, when the player dies because of a misstep on their movement, it will usually respawn back at that spot for them to try again, and not a few platforms back. In that sense, spikes act more like a "quick" restart if we design the levels accordingly. At level 3 for example, it's true the spike placements rarely provide an additional challenge for the player; their purpose is therefore to put the player right back at the challenge as soon as they fail, so the player has shorter feedback loops and can quickly learn how to overcome a given challenge.
We also forgot to mention the game progression in one important regard: the Gretel level 3 is one of the last levels in the game, so we expect the player to have more platforming experience at this point. With Gretel's story, we want to further test the player's skill, and so we are designing many individual "challenge rooms", with some requiring the player to complete the room in one full go, without checkpoints in the middle (think for example Celeste's "Celestial Resort" levels). In that regard, spikes let us enforce that vision.