If you don't have a quest board, I suggest building one as soon as you can. If you do, then do a quest every day, and ask your guildmates to do the same. It is a very slow process. I'd recommend not going for the ruby-intensive constructions right from the start. Several buildings either don't cost rubies or only cost a very small amount for the first few levels. I would not expect to get a castle upgrade with a relatively new guild -- most guilds that have them have been around for quite a while.
Teardrop rubies help significantly more than the quest boards. I would never suggest anyone rely on these quest boards to help their castle progression as the rewards from them are so incredibly and insultingly insignificant to the cost its akin to making deposits at the bank 10 cents at a time and expecting your savings to see a change.
Lets look at the numbers from someone who's terrible at math:
10 active players, using the 3rd guild quest board every day (yielding 10 rubies per turn in, 100 rubies a day) it would take:
4 to 6 days for a LVL 4 building upgrade
Slightly over 2 months for a LVL 5 building upgrade
Almost half a year for a LVL 6 building upgrade
three quarters of a year for a LVL 7 building upgrade
Total time to get 1 building to LVL 7 the old fashioned way: approx. 1 year 4 months.
To get the first wall or castle structure upgrade the old fashioned way: approx. 3 years 11 months.
To get the 2nd wall or castle structure upgrade the old fashioned way: approx. 7 years 10 months.
Total time required to get all castle wall & structure upgrades the old fashioned way: 23 years 5 months.
... So in other words, if you're going to put effort into the ruby game, it'd be best to focus on the Andor Training range for the teardrop rubies, pay attention to festival events for when they offer additional rubies as well, and purchase teardrop rubies off the auction house.