Here are some suggestions that if implemented will help immerse the player in the world of Kingdom Rush.
Intro Story
When the player starts a new game, a short story will play introducing the world of Kingdom Rush, explaining why the kingdom is under attack, and what the player's mission is. A human voice-over would work well here.
Kingdom Rush Origins
Where and when is the world of Kingdom Rush? Is it here on Earth, taking place now? Or was it a long time ago in a galaxy far, far away? What region of the main map are we seeing? Is it "the whole world"? Or only a small region of a much larger world? And who are the people who live here? What races exist and in what regions do they live? I think a few short sentences answering these questions allows the player to know as much about the people and land he has been tasked to defend, as he does the enemies and their locations. To add depth, consider adding some of the citizens to the actual campaigns as on-lookers of the battle.
Tower Origins
Archers, Soldiers, Mages, and Dwarfs. Who are they? What part of the world did they come from? Why is this their chosen profession? If there is a specific region of the world they come from, it would be neat to have a campaign each that 'passes through' their respective regions.
Hero Origins
I think it would be nice to have a little more information about the heroes' backgrounds. Specifically, what region of the world they come from, and what motivates them be part of your party. A bonus would be to create a special campaign for each region that requires its respective hero to complete it. Either that, or certain heroes can be unlocked after the player has helped them defend in the campaign. That campaign stage will then serve as a historical point for the hero's origin and why they are joining the fight. Ultimately, hero origins help the player better connect with the heroes.
Extended ending
I would also like to suggest extending the ending of the game to include how the world has improved or celebrated after the player beats the game. Even a 1-minute slideshow of the key areas would work. For example, King Denas had an ending sequence. How about the other heroes?
Of course, a much simpler option would be to include in a Lore book in the encyclopedia that provides a detailed account of the lands of Kingdom Rush, as well as a more in-depth background of the heroes, but I feel the above suggestions will create a more immersive experience.
Open to comments!