As a casual gamer, you have certainly come across titles that fall into the defend-your-castle category. BowMaster Prelude, by Jason Reinsvold of LostVectors, fits that description as well, but certain aspects elevate it above other candidates.
Unlike most other games of the genre, the player doesn't aim at attackers directly by clicking on them. Instead, the mouse cursor is used to shoot an arrow from your castle, applying force and angle by clicking and dragging intuitively. Always leaving the marks from the last shot behind, the game provides a convenient way to refine your shots as you continue to fire.
Between levels you get to upgrade your arsenal of arrows or you can buy the possibility to recruit troops during a stage. And here comes another twist that separates BowMaster Prelude from similar games. You earn experience and gold for successfully hitting an enemy soldier with the basic arrow. However, if you have recruited troops be aware that they get drafted out of the general populace. And they cost gold. If you wage a full-scale war it's a drain on your people, leaving you with precious little XP/gold to earn between stages (more people = more XP bonus/gold = more gold bonus). Thus it is imperative to find a good balance between recruiting troops and keeping the economy alive.
If you have bought a recruitment upgrade, troops will be recruited endlessly, unless you disable it. That's done by clicking the green bar above the unit's icon. There is also a variety of arrows to choose from. Some will do continuous damage to a soldier once you hit them, others will just slow them down. Some arrows can hurt multiple soldiers, others would do more damage to an individual.
And finally, your hero, the guy shooting the arrows, can also leave the castle. The A/D keys move him left/right. If you're close to your castle, pressing the W key will bring him back to safety.
Analysis: All in all, BowMaster Prelude is a surprisingly addictive game, given that I don't enjoy any other game of that variety. The Help system explains all functions in detail, including other means of aiming your arrows, which other players might find more handy than the standard control. It's still the classic castle defense game, but then again, it isn't.
Cheers to Vaidas for being the first to suggest the game. =)