Maybe I am blind, but I cannot find any walkthrough on the internets to look this up. So I will just drop it here, in case somebody can help.
I am playing Clash of the Dragons, just about finished the second last mission, the one with the Azure dragons. Next to the last Dungeon town, there is a black border guard that blocks the way to a two-way portal. The portal apparently leads to an island in the northwest corner of the map, and the only black keymaster's tent I found is exactly on that island. I found it by cruising past the island on a ship, but there is no place to land, so I am stuck and cannot proceed.

Can anybody tell me what I overlooked? There should be another black keymaster around somewhere, or another way to land on that island. The victory conditions state that you need to conquer all towns, and I did conquer all I found. Just this island is out of reach.
Help?

Update: I found out that there are no other towns in that map, so after conquering the last town you just have to wait for seven turns until the AI loses. Still, I am curious if there is any way to get to that island.

Gates and guards can also be bypassed by dimension door and/or fly, but in many of the maps that use them, those spells aren't available. If you happen to have one of those spells (from a previous map, perhaps) you can just cruise right over.

Then that must be one of those maps. Thanks for pointing it out.
I managed to get Fly and Dimension Door in the Wizard campaign, but never found them in the Rampart one.

If you want to get to the bottom of it, you can use the scenario editor and find out if those spells were excluded. If they were, then you know you're supposed to wait.