What is a premature town hall?
A premature town hall is a village in which town hall levels are advanced quickly, and usually with the aid of gems, to unlock a high-level feature -- often the X-bow or inferno tower.
A premature town hall is not the same thing as a non-maximized town hall. For many reasons it's often a good idea to complete every upgrade and building project available at a given town hall level. While there are reasons to do this, a base which advances with a handful of incomplete upgrades or building projects is not at the same risk to damage their gameplay as described in this thread.
What can you do about it?
First, you need to think hard about deleting your village and restarting. I know that it’s hard to swallow the financial loss, but you need to think carefully about it nonetheless. The reason for this is the hidden loot penalties which SC never explains, but are nonetheless key drivers of end game play. As you level up your town hall you incur penalties to looting villages with town halls lower than yours, while those lower town halls simultaneously get bonuses to looting you.
The end result of this is that as a premature TH9-10, you are the closest thing to fresh meat that exists in this game. Fully developed TH9-10 villages are going to salivate every time you come up in their attack queue, and even lower town halls are going to take a shot at you for the bonus they get against higher town hall levels.
Bear in mind also that the money you have invested is a pittance relative to the investment you need to create a competitive TH9-10 base. TH7 is the sweet spot for farming gold in CoC, and you have forever lost access to it.
So in short, your screwed. You’re both the kid wearing a “Kick Me” sign on his back, while simultaneously having less ability to dig yourself out of the hole you’re in. Consider carefully if deleting and restarting is a good option.
But let’s say it isn’t an option. The reason doesn't have to matter, but maybe there’s no way you’re going to lose that $100, giving up just isn’t how you roll, and screw math anyway. What now?
First, make sure you have all the builders. They’re far and away the best investment you can make with cash, and if you’re going to stick with your gimped base, you’re going to need all the help you can get. So cough up that extra $20, and buy the last builders.
Next, bear in mind that you won’t be able to defend your loot for a long, long time. Your best defense is simply not to have loot in your storages by spending it as soon as possible. Consider also that most people consider a threshold of 70-100k gold as the low end of what they’d attack depending on the base strength and troop investment. This means that considering loot penalties, provided you have less than 350k gold in storages, you’re less likely to be attacked by an equal town hall level. (Lower TH levels may still be motivated to attack you though.)
The general strategy is to dump loot as quickly as possible so that you never have enough in your storages to be worth attacking. Some people advocate keeping a builder free to dump excess gold into walls, but I’d personally just keep them all building. But to have the resources to level anything, you need to be gaining loot yourself. You’re going to have to learn how to farm.
There is no shortage of farming guides on this forum, but the nutshell version is to park somewhere between 1200-1500 trophies, and pick on undefended collectors and weak bases for loot with an army consisting primarily of goblins, giants, archers, wallbreakers, and possibly barbarians. In order to do this, you need to have a viable TH9-10 army. If you have any gems left over after your ill-advised rush to TH9 and builder purchases, this is where you should dump them – in your offense. Realistically a viable army at TH9 is a minimum of 200 pop capacity and level five troops. Offense and defense draw largely from separate resource pools, but if there’s ever a choice between them, always upgrade offense first.
As far as upgrading your defenses, your top priorities should be the clan castle, mortars, wizard towers, and Xbows. However, don’t do this at the expense of dumping gold fast. Remember that you’re going to be that prime target for a long time. When your base gets steamrolled, it wasn’t that you failed to copy somebody’s base design well enough. It’s that your base is underleveled.
Also, make sure that you're in an active clan that donates. New players underestimate the strength of the clan castle, but a fully staffed clan castle is a force to be reckoned with if properly positioned in the center of your base.
From recent experience I would know