While playing, press the CHAT key (M by default) and type any of the following word. For greater power, enter the same code multiple times. For example, enter CHUMP twice for greater speed and invincibility.

• CHUMP -- Invincibility
• CHUMP CHUMP -- Faster game and invincibility
• DANGEROUS -- All weapons
• MO MONEY -- Extra troops
• SLICK -- All power-ups
• TUFF -- Super weapons
• WAY MO MONEY -- Extra power-ups

Beat the campaign missions, then play any level you've already beaten. Press enter to bring up your weapons list and at the bottom will be the anvil weapon.