Monster Girl Assault, basically Plants vs Zombies but the plants are dudes and the zombies are various monster girls (gobbos, cowgirls, marionettes, witches, bat girls, vampire boss)
When a unit gets defeated they're seduced by the enemy unit and fuck
If an enemy gets in the castle you get a unique game over scene

I also enjoyed The Imperial Gatekeeper, which is just a hentai version of Papers Please