I think we should speed up the time it takes to build trenches to say 3-4 turns. If it is too quick the map will soon be like the western front in WWI, too slow and nobody will bother building trenches. Feel free to make the change to the rules file, just let me know and I will do the same. Do you know which value to alter?
One thing that has sprung to mind is about building trenches on city squares. Would this make them too hard to ever capture? My initial instinct is to ban this via house rules, what do you think? EDIT: Looking at your observations building trenches under city squares could work out. Something we should experiment with before we get too far into the game!
Regarding your observations. Remember this is still a playtest and the balance may not be 100% right yet. Historically the Nationalists did have an edge over the Republicans (mainly through their allies helping them rather than hindering them!). It would be great if we can try and find a balance where either side has the opportunity to win if played right.
Regarding the length of my turn. I did have the advantage of being thoroughly familiar with the scenario over months of designing it. During this time I considered strategies for both sides and read a lot about the war. This obviously gave me a bit of a head start, but as von Moltke once said "No campaign plan survives first contact with the enemy".