How to beat the game
I've been playing for a while now and can now post these screenshots of mazes that guarantee you to beat the game.
GRASSLANDS
First lets assume you can beat it on easy and medium modes - any rtard can do that.
For hard mode - you need to to be frugal, efficient and quick.
I dont believe hard mode is possible without 'juggling' - making two exits possible and blocking one off at a time so the enemies have to go through your path twice. The furthest I ever got on hard mode without juggling is level 85.
The best tactic that I discovered is to build the tesla towers AS QUICK AS POSSIBLE. Start by building three minigats, and upgrade two of them. Next place a goo shooter. By about Round 6 you should start saving for your first tesla. Place it ASAP - you should definately have one by round 12. (You can see the first two upgraded gats, goo and third gat by the left hand entrance.)
Then you can start building the double-ended maze using un-upgraded basic gats throughout. Look at the pic below and use that maze. The 'juggling' part of it starts at the bottom left. The very bottom gat is the one you can sell and then replace each time you juggle.
NEVER build missile launchers - they are too weak and aren't worth it.
You should always aim to have at least one tesla for every 10 rounds, so by round 80 you have 8 or 9 teslas. Upgrade these as soon as you can. As you can see I had 12 fully deployed teslas in one long 'death row'. Surround these with intermittent goo's as you progress (you can place a gat initially to form a solid wall, but replace ASAP with a goo)
Clearly this method works - but isn't perfect. I still let 17 bastards through! (Juggling with the fast motorbikes is hard - and dont get me started on the helicopters)
CROSSROADS
I didnt play crossroads at all until I'd beaten the game on grasslands.
I finished easy mode on my first try, and then medium mode straight away too!
The tactic for crossroads is different to grasslands. Instead of making a long band of death across the middle, you have to make a circle of doom right in the centre. This way enemies from above and below get pwned by your set-up.
I've posted screenshots of easy mode (NOTE: I had $386 left - didn't even need the money - and no-one got through) and medium mode (VICTORY!) - where I used pretty much the same maze give or take a few gats.
Start of by building a link from the centre top entrance to bring the enemies down to the left hand entrance. Do this immediately after you've built a tesla in the center.
Once you've got both lines of enemies arriving from the left, start by building missile launchers in a large circle around the center, leaving space obviously for the path that will develop.
You dont need many goo shooters as you dont need to slow too many things down - just 6 or 7 pointing towrds the middle will do.
Tesla's here are not quite so crucial - but you'll still need to use them - try and have a new tesla every 15 rounds. I think I ended up with 10 tesla's and 8 missile launchers in medium mode. Make sure they are ALL maxed out and upgraded before you build a new one.
Finish the maze with some basic gats - and you're done.
I haven't tried hard mode crossroads yet - but I will post another guide when I've beaten it - shouldn't be too long.
HIGH SCORES
Grasslands - Hard - round 100 - 718275
Crossroads - Medium - round 100 - 500350