With Stronghold, Melanie Rawn takes the characters she beautifully established in the first three novels and has them face off with a brutal new threat that fits with the established universe while also feeling fresh and new. 

Though many of the enemies and threats that hindered Rohan, Sioned, Pol, and their family and friends have been dealt with, not everything is perfect on the Continent. There are still other rivalries, personal difficulties, and the natural complications of ruling for the characters to juggle. Those complications are exasperated by the invasion of the Vellant'im who are pursuing vengeance for wrongs done generations before. There are many payoffs to dynamics, both good and bad, set up in the last trilogy as some characters side with the invaders for their own ends while many have reason to remain loyal to Rohan, Sioned, and Pol due to decades of earned loyalty. In addition to that, even allies have different views from each other on how the invasion should be handled, deepening the conflict even further. The way Rawn blends together the established and new conflicts makes the story feel so real. 

While the previous three novels put the main characters through heavy struggles and featured major character deaths, Stronghold felt even heavier. I can't tell if the tone is darker or if it's because I've read through decades of these characters' lives at this point, so everything that happens hits harder. 

I wouldn't recommend starting the series from here even though Stronghold is technically the first novel in the Dragon Star trilogy, which is the sequel series of Dragon Prince. Each book builds off of the events and character arcs of the other. But the books leading up to this one are nearly as good.