Use the tunnel in the royal crypt to capture Fort Morvunskar. Remember that our purpose here is not to hold the fort, however. Just to get the soldiers out of the way long enough to accomplish our true goal. Get that through your thick skull, for everything else depends on that.
The pretender Jorunn's soldiers will come in force, and when they do we must fade away. Our prize is buried beneath the fort. Taking Morvunskar was always just a means to an end. Hold it for as long as you can and then retreat to the burial vaults beneath. My traitorous brother will never suspect our true goals here, and his soldiers will never find the tunnel in time to stop us.
Make sure you keep any important relics you find locked up tight. Never trust anyone where treasure is involved. But our primary goal must be to secure the remains and transport them out of there as quickly as possible.