By definition, a castle defence game is static.
That's why castles went out of fashion. By the time gunpowder had been invented, anything that didn't move was going to get blasted by large numbers of lead balls.
To that extent then, as with so many game genres, with castle defence you're managing the process of inevitable defeat. As you get better defences, so the attacking waves become larger and more powerful. Eventually you're overwhelmed.
This is the case with Army Defender, which quickly ramps up the number of enemies you have to deal with, although it doesn't compensate you in terms of new super weapons. You start off with a good selection, however.