****If you find the achievements for Advancing your settlments through the ages are nnot unlocking, try clearing the cache or alternatively uninstall the game then reinstall it, this should unlock any stuck achievements***
To advance to the industrial age requires $6,750,00 or $6.75 million and requires 6 days to complete. To purchase the coins required to upgrade would cost you 2200 gems which is ridiculously expensive. So unless you are willing to pay which I do not recommend, you will need to grind this out by maximising your time and resources.
You should have focused on your defences and barrows in the earlier ages so now I would recommend upgrading your production centres i.e. iron forges, water mills etc.
While the battling system is still the best way to acquire wealth it should now be taking around 15-20 mins just to have your forces ready to attack, so having an alternative source of revenue is useful. Also buulding upgrades now take so long that you probably only check in on the game rather than sit playing for hours.
Make sure that your base layout is efficent, utilising every single section of wall ccan mean the difference between someone swooping in and stealing your money or you kicking their butt. I would recommend selecting CLAN on the main menu, then go to Ranks, select the top clan which is currently Battle Ages, Now click any member to view their base, this should provide you with inspiration on how to ayout your base. The worst layout is to simply build a big wall and have all your building inside,your enemy only needs to smash down one section and their army is then free to rampage. Instead construct your base in grids.
Make sure to research at every opportunity. Although the research is expensive it makes a massive difference. I have two research buildings and was able too rapidly upgrade my forces. Prioritise your research eg dont go for the cheapest research option go for what you will benefit from the most. (The Thief is great for smash and grab missions).
pgrade your walls. Again although very expensive this makes a difference to deterring enemy attacks and winning attacks. This is essential if you are aiming to store almost 7 million. You will be a huge target and having weak walls will make you an attractive target.
Finally. Do not waste your gems, the temptation to finish constructions in the last hours is sometimes irresistable but utilising their use is important.