First of, sory zup if i don't pay: i can't. but you made a great game, and its great fun. Also, the interaction you have with all the players in the game is great: i fully support your project. however (lol of course...) some things (i can understand why for some reasons you would find it unpracticall to add them), i think should be changed. I will post my ideas in different posts too (i always wondered why others dident do that, but i dident dare do it myself: thanks d93 gunz for leading the way).
#1 Protecting the zeplin is at least 50 percent harder than taking it down. Somthing should really be done about that. I think the reason is that the zeplin slowly goese over the enemy base meaning that the enemy just has to fly up and bring it down, while you fly all the way across the map loosing ENOURMOUS time. Some solutions to this could be
A) Make the location the zep is destroying other than the airport at an equaller distance to each bases, for example, lower far left, or higher far right.

Much more of a fictitious and complicated, as well as maybe just inversing the problom, make tourets on the zep itself that you can spawn in and use to shoot at enemy planes. Of course they will have to be shootable just like the engins so that this solution doese not inverse the probloms. (of couse this is not verry educationall, and i would rather opt for the first option)