By attacking another player in Monster Warlord, if you beat a player in a battle, you will take a percentage of their gold stocks from them. To prevent players from stealing your gold stocks, you will need to place them in the bank. If a player attacks you and wins, they will steal gold that you had on you, but any gold in your bank will not be taken.
To access the bank, start from the home page and click "Options". A "Bank" option should be shown on the menu it provides.
Depositing gold in the bank does not always help though. When you put a deposit in the bank, the bank will charge you a 20% processing fee (so 100 gold will mean 20 gold is taken, and 80 is put in the bank). This processing fee is lowered as the player's VIP level increases. Withdrawing gold from the bank does not require any processing fees.
The bank is required for a player to instantly heal themselves if they do not have any Charge Heals, as the healing fee will take gold from your bank and not what you currently have on you.