As the Tsar is boasting, one of his high ranking officials walks over and buries a shank under his to cage and into his lungs, then runs away. The knife has the symbol of a Sicarii assassin. (This is fake btw)
Underground fortresses? Kek. You realize that they could be starved out extremely easily, and an assailant with the proper siege equipment could literally bury you alive.
Because those rivers totally exist at every spot they randomly decide to build a fortress underground. There's no way for them to know where the underground rivers are until they dig the actual fortresses.