Sir, We Have an Orc Problem

Tower defence crossed with an incremental: thousands of orcs, pouring around chokepoints while you place artillery and drop nukes into the thickest part of the flow. The upgrade tree is the whole thing, turning each wipe into a slightly less humiliating next attempt, and it has the unpretentious pull of a great Newgrounds game. Hard to put down and doesn't overstay its welcome.