The game wants you to get all four portals down AND survive the wave of enemies that spawn after you do.
Every games a little different but -
TIPS:
Spend the first few days hiring archers and expanding either left or right.
Herd deer into the camp for extra coins.
Get a Farm going with 4 farmers by day 8-11 - This is your primary source of income for the rest of the game.
Two farms? Sounds good but dont overcommit, thats another 14 gold to make the farm.
If there is enough space Ive seen 7 farmers on the same "hut".
Get knights going by day 15-18, and start sending out expeditions, I like to send them out right when the last enemy of the night is dying to your archers.
Archer statue buff is a must before your knights go out. It looks expensive but your archers can hit like champions afterwards.
Give your knights extra gold (Up to 6 or 7 pieces each) this gives them extra health!
In a previous playthrough I defeated all four portals on day 25, but didnt clear the wave until the next day.
Didnt Count.
Also Ive read somewhere that it only checks to see if you have won ON day 25 not halfway through it, so Finish this one BEFORE day 25.
So 4 expeditions (If youre lucky) 2 on each side, should get you a victory.
If you are getting overrun after defeating a portal, you dont have one of three things.
Archer Statue Buff
Enough Archers
Tall Stone Walls
DEER HERDING REALLY, REALLY HELPS GET EARLY GAME GOLD.
Example of a sad loss - Day 24 last portals down, got overrrun cause my archer buff wore off and my guys couldnt hit the big dudes that spawn extra dudes. (Buff lasts "1 Day")