"The score often doesn't matter." Often, but not always. Risk Exposure, I finished without worrying about the score. Roundabout was a royal pain, though, as I finished it several times and got scores with less than 2,000 points shy of gold.
I tried building a maze in the square on the west for Roundabout, but it never worked out for me. What I ended up doing was this:
Start by placing an inferno on the right-side access to the core, then a gun and a temporal in the middle (forcing all aliens to access the core on the left), then a gun and inferno down the center of the next 3x3 square to the south. That's all the money you start out with. Highlight the remaining center square and place a gun the instant you get enough resources for it. Then, up the ramps, you're going to build four towers across left-to-right to ultimately guide aliens to the roundabout -- but before you build all four, you'll need to build two missile towers for the air units (first is Wave 4 and only needs one missile, but Wave 5 follows right behind and needs 2 missile towers) -- place them in the south and then the north of the large, hole-ridden square in the west.
That's how it starts. Ultimately, the path I finally had them take is roughly pictured here: http://picasaweb.google.com/lh/photo/81BLXao6jNOIvS5IZm4RVQ?...
Upgrade towers to yellow first before worrying about completing the path; you need the firepower to keep the cores from getting carried out of control. Some of the towers I have pictured, I didn't place until the very end. Also note that the last flyer wave is 23. Once the flyers are all gone, you can sell off the missile towers and replace them -- I sold the south tower and replaced it with a cannon, and the north tower and replaced it with a temporal. (Make SURE all the flyers are gone before you sell that north tower; more than once, I had to back up and do it again because I failed to notice the last flyers circling around the east edge of the map.)
Incidentally, they're discussing this on the Defense Grid forum here: http://defensegrid.hiddenpath.com/node/1007
Nice to know I'm not the only one who thinks that last wave is brutal. My strategy (much like everyone else's) seems to fall apart when the Decoys and Rumblers race in. What I ended up doing was following the last Rumbler in with my cursor, and when it crossed paths with the first escaping Rumbler (or the first one with significant health left), I fired the laser (almost always about where I indicated on the picture). Even then, I must've backed up a dozen or more times as a Rumbler would zip through.
Remember you MUST hit back before the round ends. Campaign mode doesn't let you "replay from last checkpoint" when it shows you the score (which is infuriating when you're only 200 points from the gold).