If you've never been plundered before and you are at least in Iron Age (the 2nd part I am not 100% sure of - maybe someone might be in EMA and never been plundered), then when you are attacked AND plundered - it triggers the quest. Otherwise you have to wait and get bps off of someone who gets it in a questline or someone who has got bps from someone who has done it from the questline and has built the Tower.
And the gb does cost to build -- fps. I got it thru putting fps down for bps, but a guildmate of mine in beta got it thru the questline. A bonus is it's size, the fact that it is upgradable to get more goods, and the fact that it isn't plunderable.