The Hidden Planet: The chords you use are reminiscent of Joe Hisaishi and Studio Ghibli. The progression of this track is its own story and I like the building tension with the climax and resolution. This is great story-telling through music.
The Unstirring Spring: This definitely achieves the goal of setting a scene in which life is present but has not yet awoken. I like your stylistic choice of using a progression that never seems to resolve and keeps building tension as if anticipating someone emerging into the open.
Sun-drenched Plains: Something about the clarinet just has a warm feel that fits well with Summer themes. I like the contrast between the A and B sections that remind you that even though things seem familiarly warm, you are still on an alien planet. The outro is a nice touch, like a peaceful ending to a fun summer day.
Fiends in the Leaves: The classic Pokemon-style battle intro immediately told me it was time to put my game-face on! The musical style shifts are an interesting idea. The melody and its variations are well-written and has me singing along with each repetition. I'm also a fan of your pun in the description. :)
Glacial Chill: The tubular bells are awesome in this track. I think you nailed what you were trying to achieve. It has a stark quality to the sound, yet it still feels warm as if not completely devoid of life. The descending chord progression also fits well with the idea of falling snow softly blanketing the landscape.
Overall: This is a very creative use of your existing ideas to expand and create something new that works very well even on its own. You have shown that a solid foundational idea is one way to get a step ahead of those without a solid idea of a unified theme. I think your skillful harmonic dexterity is certainly a strength of your music and allows you to evoke emotions in your story-telling and not just follow a formula. I also appreciate the creativity in creating multiple types of tracks while efficiently composing for each of the four seasons; you have cinematic music, ambient music, a battle theme, exploration music. All the pieces of a game are here! Thank you for sharing and keep up the good work!