For those that are playing the game again, you'll find that Core Sector carried over some new stuff from Ghost Hacker 2, to this one. Some bits I noticed are some of the graphics and animations. Some of the enemy layouts and solutions changed as well. (Such as the final level and Monster Mode now has some difficulty challenge bit.)
You guys do know you already 'reset' your upgrades at the upgrade screen. You also can HOLD 'SHIFT' or 'CTRL' when left clicking to downgrade stuff and get data cores back.
When you do challenge mode, make sure the towers and upgrades you are forced to use, are maximum upgraded as best as possible.
(Please hit the + button so others can see this.)
When you beat the game, you unlock Challenge Mode, which allows you to claim those extra data cores you couldn't get. This mode is the same as Hard Mode (Hard Mode - Enemies have 25% more health), BUT limits you to only use specific towers and upgrades. These towers and upgrades are locked and cannot be changed. (You can still upgrade them with data cores.)
For instance, level 1 gives you only 1 specific tower and 9 specific upgrades to use. Level 3 gives you all 9 towers and a script, but no upgrade icons for the towers.
If you find a level too difficult, remember that if you aren't using specific towers/upgrades/scripts, you can downgrade them, and move the data core points around to make the towers/upgrades you are using more powerful.
A good tip with the Glitch enemies, is to make sure your towers are not within their range, otherwise, they could zap it, forcing it to reload. It's the same as if you moved the tower to another spot. Use the 'Range' upgrade, so towers can safely attack. Remember that you can now stack multiples of specific upgrades now too. (Like a tower that only holds 2 upgrades, could hold 2 'forks' or 2 'ranges', etc.)
When you die, your other survivors DO NOT RESPAWN with you. :| I am afraid I cannot give this anything higher than 2 stars until some issues are resolved. It becomes IMPOSSIBLE when you do not have other survivors with you.