[Verjigorm]

Ok... You said they're currently laying siege to one of your castles, though, correct?

The first problem is that you didn't know they were coming. In the future, deploying more scouts/spies/etc. will alert you to your opponent's strategy in advance and you can be prepared to counter it in advance.

Secondly, the only tactics which are going to be effective now that he's invading should be based either (a) on what you already have augmented by (b) what your stored resources can provide.

What types of mages do you have ready for summoning right now?

What is your present gem stockpile at especially targeting the gem stockpile which you can consume most rapidly (which will probably be air, earth or blood).

What are your current research levels especially Conjuration, Construction, and Blood?

What types and sizes of armies do you have on the ground? Remeber, if they don't have magical weapons, they will get slaughtered by ethereal undead.

If you don't have a lot of D magic, then a "horde of wights" might not be your best bet. The Spring Hawk is ethereal and has storm power. It can't counter Darkness, though, so depending on how many death gems he has, you may have to harass him for some time to get rid of them.

You will also probably lose that castle. I would recommend burning the lab if he's deep within so he can't replenish his gem supply without replacing it.

Consider using a distraction... Even though your army is weaker than his, he is probably well consolidated I'm guessing. Send your forces to pillage his lands unless he's using a scorched earth strategy. If he has a lot of un-castled temples, go forth and burn them down. Don't siege his castles, just keep moving your army around setting tax rates at 200% until he is forced to divide in order to deal with you. The money you make in the effort will compensate for the loss of the castle. If you see some low-hanging fruit (like a poorly garrisoned castle) while ransacking his country seize it and if you cannot hold it, burn it to the ground.

Essentially, you want to make it cost HIM more to take the castle than you lose in the gambit. It is not worthwhile to dispose of your own armies to take his out if you do not have the resources to crush him. Preserve your units and use them in endeavors where they do not die in any great measure and try to buy yourself time to mount a strong counter-offensive.