Main Menu: Between the bookends of high-intensity in the intro and outro, you have a very relaxing song with a memorable theme. I like your orchestration and think you have captured a sea-like sound with the low brass drones and the pulsing strings feeling like the push and pull of waves.
Alien Oasis: The surprising non-diatonic chords in your B section highlight the strangeness and alien-ness of this new area. The mallet instrument helps make this feel like an underwater environment and I think that the long brass notes help with it as well. Based on your description, I was maybe expecting more creepiness from this track, but instead I was pleasantly surprised to find it filled with tranquil wonder at the strange new place.
Frozen Core: Low marimba is definitely an icy sound in my opinion. The low brass conveys the enormity of the fortress and the descending trumpet melody conveys the shrinking feeling the protagonist feels when seeing this strange new civilization. These are three songs that are well put-together and were nice and relaxing to listen to while imagining this alien place. Nice job and thank you for sharing!