How common historically were secret treaties like the marriage pact for Viserys and Arianne? It makes sense that you would want a heavy hitter with at least a veneer of impartiality as witness for an agreement like this (thus the Sealord rather than some random clerk from the Iron Bank), but were witnesses more likely to be strictly impartial, or were they usually involved to some degree? Thank you!

Secret treaties were absolutely commonplace for much of recorded history – hence why it was such a big deal when Woodrow Wilson put banning them as one of the first of his Fourteen Points during WWI . According to Chad Khal, there were no less than 593 such treaties made between 1521 and 2004. 

As for witnesses, that’s a bit more unusual. In no small part because they were so common, states just treated them like normal treaties, requiring only the signatures of the parties involved. You’d hardly want a lot of witnesses to a secret treaty, lest you compromise the treaty’s secrecy.