Ansilvund:
Advantages: Light cover on the outside, easily fortified within with runes and lightly armored soldiers, nearby Shor's Stone for light reinforcements, somewhat close to Windhelm for heavy reinforcements and supplies, and as with many nordic ruins, plenty of space on the inside to already have frozen or salt-preserved food supplies ready.
Disadvantages: Somewhat easy to siege, though nearby steep mountains provide some cover, difficult to retake outside position once soldiers have been forced into the ruin itself due to the area of entrance being effectively a death pit, nearly impossible to perform guerrilla tactics from for the same reason, and while reinforcements from Shor's Stone will be quick, if a larger force is necessary for helping the position, Windhelm forces would take a considerable time to get there.
Defense: Position a few archers as guards (preferable to mages due to their ability to hit from corners and with arguably more accuracy at distance) and keep the rest of the soldiers within the ruin. Use spells to hold the opening as long as possible; destroying the sole entrance would nearly obliterate this position in and of itself in the long run. Keep heavily armored soldiers by the door for this part. If the enemies break through, the best defensive strategy would be to scatter groups of lightly armored warriors along the path; with the crevice-like areas within the ruin and the generally open space, mobility is more important than standing power. Keep the mages barricaded in the innermost room for a quick burst to repel invaders should they get so far. From there, either hold out for reinforcements, since the fact that they're attacking means that they haven't collapsed the entrance, or use conjured minions to repeat the earlier defensive strategies.
Attack: Immediately swarm past the light cover and deal with the archers ASAP. Position archers and mages around the entrance to bombard all but the mightiest who step out. Take the opportunity to set your own positions in a similar manner to the previous defense's to prevent reinforcements from getting through too easily. Then use assassins or heavy soldiers to try to break through the guards right behind the door. Guerrilla tactics would be difficult from this position due to the likelihood of a cluster*bleep* upon entry into the ruin. Therefore, the only real solution is to try to break through in the conventional way. Then it's a simple matter: Either proceed with archers and heavily armored troops to try to pin your opponents down and destroy their supplies, or more simply, hold (or if we're speaking in non-game-possible terms, collapse) the entrance and try to wear out their supplies. However, this option leaves the possibility of reinforcements showing up in the long term, making it more risky.
No escape routes will mean the defenders, if overwhelmed, will have no hope of escape. This means that the defenders will fight with much ferocity - it is a main idea behind the death (aka desperate or killing) grounds in Sun Tzu's Art of War.
Attackers should not engange this type of enemy unless the fortification is lightly garrisoned (then it should be attacked at its weakest point). Offensive plan would be to cut supplies, leave an escape route for the enemy (defenders), and wait. At some point, when supplies have dwindled, they will seek escape or surrender.