try to find a dvergr underground tower. these have 2 floors. 1 is underground. enemies simply cannot walk down these stairs. this makes portals down there pretty damn safe.

another great choice are the small bunkers near dvergr piers at ocean sites when they spawn. easy to conquer as they have like only 2-3 dwarfs at most guarding them. and often carry a sap collector tool on top. oh and they come with doors installed. bring 2 iron and make a stone cutter. repair the bunker and now you have a good backup portal base. AND a decent spot to leave the boat. tough park it a bit off shore to be safe. so you can swim to it atleast.

then venture fourth and explore the biome slowly.
the mistlands i plundered are now full of holes instead of buildings and structures. kinda like planets the borg have visited....
my precious marble.... i need MOAR PRECIOUS.....