One thing I love about this book is that it fits into the "historical" subgenre of "Doctor Who" stories, but it's set in the future and involves a battle against alien monsters. What makes it a historical is that the Doctor is familiar with the situation as a historical event, and he wrestles throughout the story with how he might prevent a terrible war crime from happening, and whether he ought even to try. It's a terrific concept, but difficult to pull off, and Lyons does a fine job.
I also like how this is very much a war story. "Doctor Who" stories often feature wars, but usually it just uses war as a setting or a backdrop for a tea time adventure serial. This story is different, and it's both fun and disconcerting to see the second Doctor, Jamie, and Zoe placed in a situation where their usual adventure serial tactics are inadequate as well as inappropriate.
There's a certain school of thought that Past Doctor Adventures should feel like they fit with the television stories featuring the same TARDIS team. That's fine, but this book doesn't do really do that. Oh, the Selachians feel right at home alongside classic monsters of the Troughton era, such as the Ice Warriors. But the story constantly subverts the easy morality of a typical humans vs. alien monsters story, and the contrast is extremely effective.