Minecraft block placing/ block building - how to?

Let's say I have a strategy game with a simple terrain I've created in a modler. The player can move around the camera and he can select units. Now I want to implement that the player is able to "build" his own castle or something by placing "blocks". I can't use the normal cubes or a model cause i think if the player uses many cubes it will lag(or minimize the framerate). Have someone an idea or knows a script/sample that does somethink like this? Thank's for your answers...