I'm kind of curious why your casters didn't buff up your fighter in turn. Otherwise I agree with Pennywit, the GM should probably have done some adjusting to make things more interesting for the rest of the party.
As for your army, that sounds like your enemy taking advantage of poor preparation. Have you guys not been maintaining your military resources after the events of Book 4? Also how much downtime was between books 4 and 5?
You definitely should be focused more on defending your kingdom as best you can rather than going exploring. If you can't fight back against the opposing army, since you said you're unable to maintain your own, you might want to think of alternate combat methods. At your level, you have options - teleport, wind walk, etc. - that allow for long-range travel that doesn't risk your party getting overrun by the enemy army; locating and removing the opposition's leadership should probably be pretty high priority. Once you've managed a way to stop the incoming assault, and made sure your kingdom won't get overrun, then you can turn your focus toward whoever's behind the attack and dealing with them, and hopefully in the meantime your kingdom council can deal with building up resources again.
Also it's kind of useless to go hexploring if you don't have any BP to claim new hexes with. You're better off using what spare resources you can scrape up to build new farms or other things that actually produce BP, so you can make back some of what you've apparently lost.
Seriously though, I have to ask - between the magic item economy and building up farms and such, you should be able to build up more than enough stock BP, unless you've been building yourself down to zero every month. How'd you get your treasury so dry?
I finished Kingmaker last saturday (we skipped the 6th book because some players are not found of high level play).
The tournament felt a little unexploited but have some great opportunities. The different contests require to weight reward vs risk to maximize your score and between them you can prepare some cheats or try to prevent others to do this kind of things. Our paladin king won the lumberjack, my character (elven rogue/fighter) did the archery test (finished 3rd), the boasting contest (finished 2nd, too bad our bard was absent) and won the drunken joust. He also took opportunities to talk to other contestants in ordre to recruit NPCs for our little kingdom.
But as I said, the tournament feels unexploited because it lacks minor events and if you run it basically it's just a succession of contests.
The Fort Drelev attack was not a surprise, we considered ourselves already in war against Pitax and our defenses were ready. The battle was harshed but ended with our victory. The problem is that we didn't really like the mass combat rules. They are not bad but the PCs have very little influence on their units and in the battle and 2 of our players didn't like that.
So we completely skipped the exploration and battles. We just went to the Abbey before ending the war in a good old time D&D fashion => commando attack against the leader. We precautiously planned our attack, sneaked into the palace directly to Ivoretti's bedroom and defeated him.
The next morning, we announced the death of Irovetti to the people and that now my character was the new ruler of Pitax when some Ivoretti's companions (the oni and the gargoyle) tried to avenge their friend, without success of course.
we have had a habit of spending almost all of our BP making new things every time, I guess we were a bit dense and didn't realize that the game was building towards something military-wise.
Yeah, the attack at the beginning of Book 4 was supposed to be your wake-up call of "not all your neighbors are friendly, if you don't have an army you should start working on one".
You'll basically be able to form armies of them as well. You might even get a discount on BP for having so many followers.
Perhaps you can work with what you have got.
"There are alternatives to fighting."
I would consider going after 'the source'. Do some investigating and some divinations. Call in some favors from your allies and work to set things right on a personal level. Being that your kingdom seems to be a peaceful one, perhaps you should turn to your neighbors as well. If this King betrayed you and your kingdom I'm sure yours isn't the only one he has done it to.
No offense, but what kind of nation gets attacked and then never thinks to plan to thwart such events in the future?
A group of people go from surveyors and explorers to nation founders. If these people never think to protect what the have they don't deserve to have it.
"You have what you hold" is one of the laws of the River Kingdoms.
Perhaps your group is a bit used to being led by the nose. Sorry to hear your campaign fell apart because of this. I'd like to think the heroes you were playing might still care about their kingdom. They might even donate some of their belongings to fund an army charged with pushing back the invasion.
As far as your spy.... Do you think that Russia had spies in 1941?
While I kinda agree about the GM not at least dropping some hints about what's coming (and a few more problems as well), that's still only him compensating for your own failure to read the very obvious message that was the entirety of the previous module and forgetting that the point of Kingmaker is that you are more than just a group of random Adventurers this time around. Even if you thought it was just background fluff, the idea of raising an army in the face of the guy next door getting invaded is just common sense. That is NOT "sniping," that's an act of war.
I'm going to let all the hate flow here:
It's everyone's fault!!
**dodges the rotten tomatoes**
The DM should have made some knowledge available indeed, and should have worked to give some clues about impending danger!
The players should have started thinking as rulers instead of adventurers, and prepared their country and cities properly. Hell, even invested their own money into hiring a company of mercenaries to protect their kingdom?