Anyone have some advice about beating level 5 in the campaign mode? That's the level where you have to send your army across the river to defend the capitol. This level is a nightmare. I'm supposed to build an army to defend the capitol, but I can't even defend my own side of the river because the enemy keeps drowning me with knights and halberders. How the heck am I supposed build an army for the capitol when I can't even defend my own camp?

I had a hard time in this level.
You need to construct A LOT of stone towers and walls around the city and make an army of archers there.
While you are doing this, go defending your city in the 2 bridges until the atack in the capital starts. When that happen, if you made enough towers the city will defend itself. I know the atack will be huge but thats why you should prepare yourself for a massive battle.

After that, focus the war in the enemy beyond the 2 bridges and crush them

(sorry for the english)

I have another little tip: destroy one of the bridges with your heroes (wizard and pikeman): this way you won't have to build twice as much defenses on your side of shore. Just don't forget that you will have to destroy the northern camp in order to complete the mission so demolition of both bridges is out of question.
Oh, and yes: this is the sixth nission, not the fifth.

There is quite easier way to do it, as soon as game starts send heroes to destroy bridges, later you can use trick to kill this enemy, after defending you can aboard army and go to upper corner of map, as you can see clifs in theory don't allow to disembark army to the ground, but you can manually point disembarking option a little further than cliff resulting in "teleporting" units on enemy side, you can use it to clear place and by using peasants you can build FOB there and slowly destroy enemy.

In case anyone comes across this, you could also try:

_ Restart the game on Easy difficulty.
_ Just let the capital fall. You will have to play an extra level after this, after which it proceeds as normal.
https://www.gog.com/forum/tzar_the_burden_of_the_crown/did_you_know_there_is_a_hidden_level_in_the_campaign

Also, whatever you do, try and level up your heroes in early levels as much as possible. They will become formidable later on, which will make later levels much less frustrating.

I just passed this level on hard without a problem with a little "cheat" - just send both of your heroes and single worker to northern enemy base at start of the map. Pass by the towers by going near the river - you will receive few shots, but that's nothing for you heroes. Then continue to north border of the map, turn west and kill every enemy worker on the way. Build outpost somewhere over enemy castle and start killing everything, healing from time to time in the outpost.

In the end I had no problem with northern base at all.
As for the defense of the capital, I made 6 "bunkers" of four stone/cannon towers near entrance to base, surrounded with stone walls, plus 100+ units - knights, longbows etc. Few barracks inside capital help as well :)