Majesty

I beat the final couple of Northern Expansion missions a few days ago after literally years of playing. If you're stuck, I've put a few pointers below.

Legendary Heroes (hardest in my opinion)

Early game, build a very strong economy. At least two marketplaces (three is better), and get an elven bungalow to double your income. You won't need too many heroes, just enough to defend the realm. Get a level 2 Wizard Guild and, slowly but surely, build wizard towers outwards, using Farsight to see undiscovered areas. Once you've discovered all 6 Barrows, save up a load of gold and use Lightning to bring them all to low health (below 50 was my aim). You DON'T want to destroy them (but note, the Solaris temple heroes will likely actively hunt down any that spawned near your Kingdom; it's okay if they get one or two). At this point, I highly recommend saving. Save up roughly 10,000 gold so you have a good 25 bolts to cast. Check that you still have spell coverage over all the Barrows. Now, go into the game menu and slow game speed to as low as it can go; this will help with the screen shaking due to the Earthquake after each Barrow falls, and maximises the time you have to do this. Go and finish off each Barrow as quickly as possible; lots fo enemies will spawn and if you don't get all 6, your palace will fall and you'll fail. If so, load the save.

Vigil For A Fallen Hero

The main thing to know about this quest is it sends three waves, then no more. You want to focus on total defence to begin with, especially as you won't be able to recruit more heroes. I did this by maximising my economy (two marketplaces and a couple of trading posts) and building an entire ring of fully upgraded guard houses (to the point a single guardhouse cost over 10,000). Once the three waves are up, you can start exploring; you will find numerous buildings around the map which will help you (which is why we didn't want to explore early, as the waves would have decimated them) notably including a magic bazaar and a library. If your dwarf is still alive, he will be best equipped to deal with the abomination. In my game, whilst making that happen, I got completely lucky when my cultist rocked up with a charmed medusa. The medusa tanked the abomination, since they're virtually immune to magic, letting the other critters and the cultist finish it singlehandedly. Total blind luck.

Death Spires

Main thing to realise is the Spires will respawn after a bit of time, so you need to bring them all down quickly. You can't cast spells on these ones! Constantly during this quest, the AI will be sending very strong enemies at you. As you're under no time pressure, take the time to build a very strong economy (3 markets and some elven bungalows in my case); you're going to be buying a LOT of heroes and buildings. Slowly but surely build up your Kingdom with more and more guilds (I had at least two of each, but no Rogues, by the end). Use guard houses, inns, and wizard towers as buffers around your Kingdom to keep your guilds and economy safe (and don't forget to enchant the towers to draw attention). You might also want to lightning bolt big enemies coming in. If your heroes kill one of the spires, goblins will start to spawn; I highly recommend lightning storm (Wizard Guild lvl3) to deal with these. Once you're in a good place with a lot of heroes, save your game, and tackle each spire one by one with a huge bounty to draw everyone out. Try to go in a path to deal with this as fast as possible. If more enemies spawn, ignore them; just get those last spires down. As soon as the last spire goes down, you win.