Make ports 1 way to others realms so that under pop realms can have chance to stop invasion or slow it down.
For example:
Alb invades Odins and take Bled and Nott w/ open port with 9fg. Mids have 3-4fg to defend but can't stop Albs, so a group or 2 can go to Alb and try to cut port to slow the down the Alb assault without Albs being able to insta-port with 9fg. Make the invaders have to suicide to defend their own land as draw back for assaulting another realm.
For example:
Alb invades Odins and take Bled and Nott w/ open port with 9fg. Mids have 3-4fg to defend but can't stop Albs, so a group or 2 can go to Alb and try to cut port to slow the down the Alb assault without Albs being able to insta-port with 9fg. Make the invaders have to suicide to defend their own land as draw back for assaulting another realm.