Let's start with the basics of the string section — and please forgive me if you already know all of this. The modern orchestra divides the strings into 5 sections, arranged from highest to lowest, as follows: 1st violins, 2nd violins, violas, cellos, basses. There aren't really any hard and fast rules to govern which section plays what other than what physics dictates... the limitations of the instruments themselves. In general, the 1st violins carry the melody (think sopranos in a choir) and the others provide some sort of harmony or counterpoint. This is a very loose rule of thumb, though, and a quick glance at any given score will prove that the other parts often carry the melody, as well. If you listen to orchestral works that features strings — which is nearly all of them — you'll develop an ear for what you can do with the string section. Or, perhaps more accurately, what you can't do. The strings are probably the most versatile of all the orchestral instruments!
Fresh composers typically have the hardest time writing for the viola section, likely for a number of reasons. First, it is difficult to hear the viola section in a piece of music unless it's been given a solo run. So composers may not "hear" the viola part in their head when trying to write. Second, the viola uses the alto clef, one of the only instruments to do so, and it can be very time-consuming to transpose for this clef. It's so tempting to just copy-paste the 2nd violin part or a transposed cello part into the clef and move on. Finally, the viola doesn't get much air time (compare the number of violin and cello concerti, for example, to viola concerti), so it can be somewhat unmotivating to write for the "stepchild" of the string section. Thus, the tendency is to relegate unexciting "filler" chords to the viola section: whole- and half-note harmonies, and the like.
The bass (AKA contrabass or double bass) suffers a similar fate: play the cello line an octave lower.
(By the by, violins use the treble or G clef, violas use the alto or C clef, cellos use the bass or F clef, and double basses use the same clef as cellos except their notes sound an octave lower than written.)
You begin your foray into string writing with a bouncing melody played by the lower violins/violas (hard to tell which). This is textured later by soaring passages by the 1st violins a la Pachelbel's Canon in D. Then some half-note ostinati while the celli and basses pizzicato beneath, soon alternating between violas/celli. This is followed by another pizzicato passage at around 4:00, then the lower violins/violas play the melody above.
Personally, I don't have any complaints. My ears aren't bleeding and it kept my attention to the very end! Admittedly it is difficult to evaluate the depth of your string writing without a score (which I understand you won't be able to provide this go-round), but I can say that from what I heard here you seem to be on the right track. I am encouraged that you gave each section counterpoint (for the most part) and not block harmonies. Strings are certainly capable of producing lush chords, don't get me wrong. However, that seems to be all the amateur composer can use the strings for and they forget about the section's extreme versatility. So good job, you!
I had no problem with the ending. It is your piece, though, so tweak it until you're satisfied.
To me, it evoked the excitement of discovery, of passing through unknown corridors and casting light on some of the world's buried mysteries. But again, that's just me.
All in all, a tasteful piece. You clearly have a knack for motivic development and constructing meaningful harmonies. I look forward to hearing more of your stuff!