Thank you so much, and that is a very nice choice for a favorite tune! The Hidden Door itself is a rather weird track. It started initially with the opening drum pulse and a fair bit of eq manipulation. By changing the frequency domain makeup of the sound during each hit, it sounds as if the drum sound is being struck from different areas on the drum like a percussionist might hit a tom-tom in different places around the rim to activate different timbres. The other important composite sound at the beginning which eventually overwhelms the rest of the track is itself rather simple. That sound began initially as a sample from a bit of sound design I did for a different project. I shaped that sound a bit, attached an arpeggiator to the midi device and then experimented around with manipulating the arpeggiators makeup with one hand while playing the synth with the other. After that, I gated different sends behind certain frequency ranges and what you heard on the track emerged from that! As the piece developed from there, most of the composition turned into freezing midi to audio, and then manipulating it as needed without losing the core sound.
Hope that helps explain the track, and I'm really glad you enjoyed the atmospheres within the score!