Network for a bank and its branches

Hello,
I am a network administrator for a small private corporate and recently I have been contracted by another company to design (from scratch)a new network architecture for a banking institution with 3 branches which all need to be connected asa WAN. Any suggestion on what is needed (hardware, software,..)for a bank in particular as I have never dealt with a banking system in terms of networking. I know one thing is that the security should be the #1 concern. Thanks