When in MS-DOS or DOS go in the C&C directory and type in "C&C FunPark". This cheat gives you hidden levels with dinosaurs and barbed wire in 2 Player mode.

Send the unit right next to a sandbag wall and then click the sell button. The sell icon will come up. Put the sell cursor as close as the unit as you can without it becoming crossed out, then click and the unit will be sold.

If you put a vehicle onto a repair bay and put the sell cursor over it, you will be able to sell the vehicle.

After building Com. Center, click on a moveable unit. Then build a structure, but don't place it. Scroll the screen so there is a building with enough cells on its right side to place the structure you have built, with one cell of that space separating the placed building and the map. Click to place the structure and move it to the space next to the map. You should see white hash marks. Then move the cursor onto the map, where it should show as a green 'move' cursor. Click on the map where you want to place the structure.

If you're having trouble versus the computer and have some extra money, just expand with sand bags. Move towards the enemy base and sell the bags behind you. When you have reached the enemy base(s), just put a plug them up by putting sand bags in the entrance of the base. The computer is so dumb it won't attack the bags and they will be stuck inside, barring that you didn't or won't blow a hole in the side of their base at any point... Then all you gotta do is make a bunch of kick ass units and build some towers outside the sandbags (near their base) and they will be completely screwed.