Hello Sir, this offer appears random, but usualy pop faster when u start to build walls or towers even gates, it last for 1 day usualy and lets say u have a few lvl 1 walls allready placed, that offer will make you a full rank 1 walls with towers gates and everything they need to be placed. you just buy and they place them for you. same if u have lvl 2 towers etc it will pop the offer for rank 3 etc, the cost is not default as it changes depending on what is your walls rank in that moment.

Thank you for the valuable information you shared. so judging by what you say there's no need to wait for unlocking fortification 5-10 lost art or stoneheart fortification lost arts to save money right?

I dont think those offers appear if you dont have the lost art discovered, its a long time since i finished all mine to be honest