I've been thinking lately about TD games, of which by far my favorite (just to lay my biases out up front) are the Bloons TD games. Not least of which because they're nonviolent, unlike 99% of TD games, which are basically built around methodical mass murder. But the Bloons games are all track-based TD games. Sometimes there's more than one path, and occasionally a path has a branch point, but basically the bloons are all on a track and you know where they're going to go. And, with a few rare exceptions, you can't build a tower on this track, so as to physically block them. This contrasts quite a lot with "open field" TD games, where there is no track; creeps are just trying to get from one side of the screen to the other, and you can physically block them with your towers. I've tried a few of these, and didn't like them much, but I can't put my finger on exactly why. So. Game design question for the group: how does this decision (open field vs. track) fundamentally change the fun factor? And (for bonus points!), are there other types of games where you could apply a track to a previously open field, or vice versa, to make a very different variant on the design? Cheers, - Joe P.S. And yeah, then there are multi-track TD games like Plants vs. Zombies, which are sort of in between. But let' s not muddy the waters too much just yet.