The Siege of Runedar is marketed as a deck-building game from Reiner Knizia and Ludonova, but "deck improving" might be a better description as your deck size remains at twelve cards over the course of play, with you ideally replacing starter cards with upgrades, then still better upgrades.
One problem, though: Your deck contains two orc cards that can never be upgraded or removed, and whenever you start with one of those in hand, you draw a card from the orc deck, which will add one or more orcs around the fortress, advance those orcs over the walls and into your living area, or bring a siege tower or catapult within range of attack.
The one bright spot in this situation? You might not see those orc cards thanks to a twist in how you handle the cards in your deck. After you shuffle your deck, you place two cards on the discard pile, leaving you ten cards that you will play over the next two rounds, five at a time. You never draw extra cards on a turn, and you won't know which cards have been removed until you pick up your second hand of five cards.
With this set-up, you know that bad things are coming, but each time you pick up a new hand, you hope for a reprieve. No orcs in the first five cards? Okay, great for right now — let's go to work! Maybe they're both waiting in the second hand, which means you'll have only three cards to play, which won't allow you to do much...but maybe you'll find fewer than two and can do even more. More than other deck-building games, The Siege of Runedar gives you opportunities for good feelings just by picking up your hand and seeing what's not there.
To upgrade your deck, you play cards while in an orc-free workshop to collect matching resources — metal, leather, wood — then place these resources on the available upgrade cards. Once you've paid the complete cost of an upgrade, any player during their turn can choose to remove a non-orc card in their hand from the game and replace it with that upgrade, which they can then play on the same turn.
Each card you play can be used for movement points in the fortress or for one of its listed powers: resource gathering, close combat, long-range combat, or digging. That last one might seem unexciting, but to win the game, you need to clear all the rubble in the fireplace room — 6/8/10/12 pieces, depending on the difficulty level of the game — which then clears one of the five tunnel pieces, with you then confronting two goblin tokens before you can start digging again to clear the next batch of rubble.
Goblins come in five levels of difficulty to match the five tunnel pieces, with you drawing two at random from the appropriate level. Maybe you'll have to fight the goblin, maybe you need to give it resources to pay it off, and maybe the goblin has brought more rubble that you need to clear. Whatever the challenge is, you probably won't be happy to see it.
Each turn, you are pulled in many directions, needing to collect resources for upgrades, to dig to clear rubble and escape, to fight orcs so that you can collect resources or not have them steal your gold, and to remove the siege tower and catapult before they cause large-scale trouble.
Combat is handled with six-sided dice, with two sides showing a crossbow for a long-range hit, two sides with a single close-range hit, one side with two close-range hits, and one side with three close-range hits. For combat, you play as many cards as you like with the appropriate symbol (close range or long range), then roll all the dice at once. You need two hits to kill an orc, which discourages you from playing one card at a time since a single hit does nothing and doesn't carry over to the next roll — yet if you overcommit on combat, then you give up cards that could be used for something else. These choices are standard stuff in games of this type, and they work as intended, with you sometimes gambling on a single roll to great success and euphoria and sometimes committing big to a "must win" situation that you do not win.
As with many co-operative games, you can lose in various ways, e.g., if the last piece of gold is stolen or all ten orcs are in play. You can fight against those two loss conditions, but you do have a definite clock in the game, that being the fifty-card orc deck. If you draw the last card, you lose, and while lucky shuffles may keep the orcs away for a while, inevitably you will draw those cards and see more forces against you.
Siege towers bring troll attacks if you don't remove the tower in time, and if you would be attacked by trolls for the fifth time, you lose. The catapult has a similar loss condition in that the fifth such attack kills you, but you'll feel the walls closing in long before that because each time the catapult strikes, you lose one of the possible upgrade slots, giving you fewer choices of how to make your deck better and possibly costing you already collected resources, i.e., time, with time being your most precious resource of all.
I've played The Siege of Runedar twice on a review copy from Ludonova, both times with two players, and offer more details of gameplay and more thoughts on this SPIEL '21 release in this overview video: