I almost made it to night 40 on my first run, but my knights all kept failing to destroy even one portal on a peaceful night.
I learned I have to start trying to destroy it much earlier. However I think it triggers a real nasty invasion every time you fail...
BTW, I found the best strategy is to slowly (in the course of 5 days or so, always AFTER you get your chest) upgrade to the 4th tier Camp (this still gives you 4 coins compared to the 5 at the beginning and the single coin you get from the castle) without actually upgrading the inner walls or towers (you get stone walls and archer nests for free). Also, ASAP build the EXTERIOR towers, they are much better at killing invaders than the interior ones.
Also, whatever you do, don't destroy the peasant camps (they leave when you cut the last tree separating from you), and keep your base as small as possible. Just 3 walls if possible (the left one just so you can park the catapult there). And recruit TONS of archers.
EDIT: Crap, I think I overdid it a bit...
There's as many on my far left wall guarding my farms, and those portals are gone so they basically just hunt rabbits all day. These guys basically one-shot all the flying thingamabobs without any casualties, and even the big troll things are killed in seconds (if they manage to lob any stones they usually just hit the archer on the top of the tower, so I only lose 3-4 bows to every horde. If they reach the wall... well, they become sitting duck pincushions)