On a slightly different note:
Would it be possible to incorporate a way for more players to contribute sapphires towards upgrades? Perhaps some form of guild sapphire bank would serve this purpose.
I appreciate that you need an income stream to keep the game running and have no objections to that. However most people don't want to spend that many sapphires at once, especially on something that can be downgraded and the sapphires wasted. So providing some sort of mechanism to allow more players to contribute to each upgrade would be helpful. At the moment the options are either 1)pull out lots of sapphires from a handful of players to achieve the upgrades and boost to get to the next safe point - which isnt an option for man. or 2)a very long and tedious grind to acquire the glory points to do the same which doesn't really contribute much to your revenue stream.
If you feel that the sapphires absolutely must be purchased and not come out of ingame generated rewards, then perhaps you could have a purchase sapphires for guildbank option rather than a simple transfer of x sapphires. Or when you purchase any amount of sapphires, you have an option - donate x sapphires to guildbank. Either of these would multiple players to contribute.
For the same financial reasons, it would seem to make sense to have more safe levels than 5 and 8. If combined with the above features, then this would give players and leagues and incentive to upgrade the fortress and would surely enhance your revenue stream rather than reduce it.
You might reply by saying that this option would provide a limited and very short term boost. this is true in terms of immediate fortress construction- however new leagues are always forming and those that already have a fortress will have an incentive to spend on knowledge upgrades so you would have some level of sustained spending.