The earliest Fortresses that were used (Storehouse, Goshen, Kingdoms, Temples) were all targets for Rubble and Dust so I think that's where the original idea stemmed from. Then we started seeing Sites and Fortresses that were having huge impacts on the game without being targetable by something like R&D (TGT, Caesarea Philippi, etc). However, we didn't really want to print a dominant in the tin sets so we had to wait until we were able to do a full sized set, which ended up being The Early Church. Since R&D was O.T. (and we knew Persecuted Church would be next), it made the most sense to go ahead and print a Fortress/Site counter, and Shipwreck was a logical choice for that.