Age: 
Future Age
Cost: 
300,
350,
375,
450,
400
Size: 
7x5
Road: 
Road Required
Benefits: Bonus rewards and Goods to the guild.
The Best Great Building
The Arc is the best great building both for player benefits and also for guild benefits. It is a Future Age Great Building but there are ways to get it much eariler and it had big advantages to do so.
Bonus Rewards
The more aluring of the two bonuses is the Contrubution Reward Boost. What this means is when you contribute Forge Points to another player's Great Building you'll get a bonus back on the Forge Points, Medals, and Blueprint awarded to you. At level 1, this bonus is 10%. It is 31% at level ten and 91% at level 90.
For example, with a Level 11 Arc, you get a 32% bonus. The first place prize for a level 6 going to level 7 Lighthouse is 40 Forge Points, 40 Medals, and 2 Blueprints. With the 32% bonus you instead get 53 Forge Points, 53 Medals, and 3 Blueprints.
This bonus can be very helpful to get extra Medals and Blueprints, but with the Forge Point bonus you recieve extra Forge Points that can be reinvested in that Great Building, making it easier to invest at the same reward level and provides a quick reevestment back for the owner.
Guild Goods
The Second benefit of having an Arc helps your guild. Once every 24 hours, the Arc will produce a certain amount of each of your age's goods that are directly contributed to your guild's treasury. At level one, it's 9 goods. At level 10, it's 22 goods, and 182 at level 90. This greatly helps your guild gain goods without having a big impact on you.
For example, with a Level 11 Arc, it contributes 24 of each of the current age goods to the guild. This means your guild is getting 120 goods everyday.
Getting the Arc before the Future Age
Step One: Get Blueprints for it
Look for guild members who already have an Arc and contribute to their Arc to get Blueprints. The Arc also gives very high Medal rewards, as an extra bonus.
Step Two: Find a Goods Dealer who is in Future Age or higher
Find people who are in Future Age or higher. If your guild doesn't have any and you have trouble finding one, ask your guild if they know anyone or post in Global Chat. Note, there is a chance the other person might not follow through so it's best to go with a guild member or someone recommended to you. The good dealer will ofter unfair trades to you in exchange for putting so much Forge Points on one of their Great Building.
Alternate Way
It costs a lot of diamonds, but Blueprints and goods can be purchased with diamonds, assuming you have at least one Blueprint.