Problem was,
@nj666's empire was already crumbling from unhappiness, so he got pushed back into the ocean.
No, this wasn't the reason for pushback.
My army outnumbered him like 5 to 1 when it arrived. I managed to capture two cities without much problems, but then he got one general and his army (which still small) healed. My army was partially injured but I still managed to push him back, killed some units and damaged most of them. But then he got another general, fully healed and I was caught by surprise as I wasn't expecting another general so soon. So I lost some units to it, but then attempted to hold these 2 cities. It was okay until he got his third general, my army didn't have time to heal so I was forced to fallback. Some of my fresh reinforcements were on their so I was going to capture these 2 cities again.
But then he started exploiting nasty city trade bug.
He captured city with his unit, then immediately gifted it to vassal ( and his unit was moved away ). I recaptured city but my unit remained in it and it was lost when he captured city again ( and again gifted to vassal ). I didn't have power to clear out his army and there was no way to hold any city without losing units to it ( while he never lost unit because city was given to vassal and his unit removed ). So I couldn't do anything there.
So, two reasons: overpowered generals and exploiting city trade bug.
You seriously claim that it is impossible to ally agains Sweden and kill it in Human vs Human game?
I seriously claim that it will only help Sweden, as more wars = more generals = more healing of everything ( and healing is not limited by units count or total HP count or anything ). So with growing army and war activity the overpoweredness will grow exponentially.
If I dont play a warmonger, I wouldnt attack Sweden.
Yes, your tactic may work.
But:
1) managing to build Great Wall ( or any early wonder ) is mostly a lottery, esp. on Emperor diffuculty
2) you won't have enough generals to build enough citadels ( unless you are lucky to spawn in area covered by mountains and tight passways )
3) you will surround yourself with cage and be unable to expand ( or at least expanding will be very slow and inefficient )
4) you will have to spend lots of industry, gold and maybe even culture ( policies ) and faith on military, so it will slow down your peaceful development. you will have to always expect Sweden attack, and Sweden is warmonger civ so it will always have many war units, so you will have to have similar ( or even larger ) number of units to hold the line
5) if somehow the 'high-security line' is broken and Sweden went through - its mostly over
6) while you are defending your territory, Sweden will conquer other parts of world and then crush you anyway
The only cases when Sweden can lose is when its:
1) exceptionally unlucky starting conditions
2) exceptionally bad skill
Even alliance vs Sweden wouldn't work as then Sweden will make more generals, so more healing will occur on both fronts, so alliance will just benefit it