Pomerania has a dense network of Teutonic and episcopal strongholds that you can tour on a single longer loop. Here are six to begin with.
Malbork: the largest of them allMalbork Castle is the largest brick fortress in the world and a must — set aside almost a full day for the audio-guided tour. Buy tickets online in season, because entries are timed.
The Vistula valley: Gniew, Sztum, KwidzynLess than an hour from Malbork lie Gniew, with its Gothic castle and knightly re-enactments, Sztum on its lake, and Kwidzyn with its famous "gdanisko" — a tower linked to the castle by a long brick gallery.
Kashubia: Bytów and LęborkThe western end of the loop holds Bytów, whose Teutonic castle now houses a museum and hotel, and Lębork with surviving stretches of wall. This is the quieter, less crowded part of the trail.
Tip: realistically you'll see two or three strongholds in a day. Give Malbork its own visit and combine the rest into a weekend loop.