Hey there!
This is a simple guide to get the 25 day achievement. It's quite monotonous, but easy to remember. You'll need a basic knowledge of the game, but you don't have to be a pro to follow it.

1) The Begining (day 1)

At the start of the game immediately hold down the ‘down’ arrow key to deactivate tutorial mode. If you don't do that, you'll be obligated to buy/build unnecessary stuff, which is a waste of coin and time. Go to the camp, light the campfire, hire the two peasants, buy 3 bows and start hiring more peasants from the wilderness to recruit archers.

2) Recruitment (until day 15)

What you should do:
Get coin from hunting archers, buy bows and hire peasants, running back and forth until you have enough archers to supply you with enough coin for a full “hire-run”. A good strategy here (when you have the money) is to wait for an attack to be repelled at night and right after that gallop through the wilderness (alternately one side at a time/day) until you reach the last camp, while hiring every peasant on the way, than rush back to camp to buy bows. (Don’t go farther than your purse lets you OC.) If you’re fast/lucky enough you can still hire from the first 1 or 2 camps from the other side if they’re close enough. Stopping at grassy valleys for a brief moment to feed your horse is advised, as well as buying the dark-brown horse ASAP. Recruit as many archers as you can until you reach day 15.

What you shouldn’t do:
Don’t build/buy anything else. Trust me, you won’t need walls with that many archers early on. Don’t even think about improving your main building/camp, because if you do, you’ll destroy the hunter field for your archers (i.e. no income). Try to avoid leaving bows in the night, because them “greedies” will swipe ‘em right from under your nose.

3) Preparations (day 15-20)

Recruit builders (8-12) and upgrade your main building and the four surrounding walls to maximum level. If you’re smart enough you’ll start with the residence, because you’ll get free lvl 3 walls at its final stage. Towers are unnecessary if you want to win fast, so don’t build them. You’ll need two or at least one large farm. Always start by building a wall beyond the mill if you don’t want your farmers to be slaughtered. A good idea is to begin construction right after an attack. When you’re finished with all of this, clear some trees beyond your borders so that your hunters may remain active granting you additional income. Finally, buy the four knight crests and build the catapults. If you give some coin to the knights you can make them more durable.

4) The First Strike (day 20)

Send attacks from both sides at the same time early at dawn. Right after you launched them, rush to the Archer Monument and light it up to its maximum level, than back to camp. A good idea is to send the second attack in direction to the monument. Hire peasants on the way just in case. If you did everything right, you should have a ton of archers to repel the counter invasions easily. If some of your knights died, buy their crests again, recruit more archers if necessary and prepare for the final attack.

5) Finishing The Game (24)

It’s very important to launch your final attack at the beginning of day 24, because if you don’t, there’s a chance that you’ll run out of time before you could defeat the last wave. Once again send them simultaneously than run for the monument and back to camp, while hiring peasants for reinforcements. The last wave will be massive, but if you built up your defences well, you won’t need to fear anything. If you want to make sure, light up the Builder Monument right before day 24.

Good Luck! :)