Unlock Field Training bonus':)
Finish field training mode to unlock the "War Room", "Campaign", "Design Maps", "Stats", and "Special Intel" options.

Quickly finish Field Training:)
Select the final battle in field training. Win this battle to quickly unlock all options that require field training to be completed.

In-game reset:)
Push Start + Select + B + A while playing the game. This may have adverse effects if you have plugged the Game Boy Advance into the GameCube.

Delete saved games:)
Press and Hold L + Right + Select when turning on your GBA.

Play in ghost mode:)
Press and Hold B during your turn.

Unlock Advance campaign mode:)
Finish campaign mode. Open the battle maps screen and purchase the "Advance Campaign" item for one coin. Press and hold Select and select campaign mode to begin an advanced campaign, with a more difficult artificial intelligence.

How to purchase Drake:)
If you use Andy to win the Captain Drake, Naval Clash, Wings Of Victory, and Battle Mystery missions in campaign mode, Drake can be purchased for 50 coins at the Battle Maps screen when the campaign has finished.

How to purchase Eagle:)
If you use Sami to win the Captain Drake, Naval Clash, Wings Of Victory, and Battle Mystery missions in campaign mode. After this is done you need to win the Rivals battle. Now Eagle can be purchased for 80 coins at the Battle Maps screen when the battle has finished.

How to purchase Grit:)
Use Max to win mission 4 in campaign mode. Grit can be bought for 50 coins at the Battle Maps screen after the campaign is over.

How to purchase Kanbei:)
Finish the campaign mode under the normal difficulty. Now Kanbei can be purchased for 50 coins at the Battle Maps screen when the campaign has finished.

How to purchase Nell:)
Achieve a "S" rank in advance campaign and war room game modes. Now Nell can be purchased at the Battle Maps screen when the campaign has finished.

How to purchase Sonja:)
First you have to win the "Kanbei Arrives" mission in less than 9 turns; the "Mighty Kanbei" mission in less than 11 turns; and the "Kanbei's Error" mission in less than 13 turns to unlock the bonus Sonja missions. Then beat the bonus Sonja missions in the campaign mode. Now Sonja can be purchased for 60 coins at the Battle Maps screen when the campaign has finished.

How to purchase Strum:)
Unlock Grit, Eagle, Drake, Kenbei, and Sonja. Now Strum can be purchased  for 100 coins at the Battle Maps screen.

Stopping the enemy from building:)
Put any troop type (or at least a medium tank) on an enemy building.