When you sacrifice a preist on your altar. You get to choose a weapon you would like to use in future games. You can save up your kills to get better things. Each kill you make will add to your level. Once you have earned all the units and win another fight, you can choose to go to rank 2, which means all of your units are gone, except the first two, but they are more powerful.
Fighting is a pain.
You build bridges around to islands to collect enough energy. The units you build just stay there and shoot whatever comes close. Its also important to protect the base from attack. You need to use your units to be able to catch another players preist and bring it back to your own base for sacrificing. A big problem is, that someone with a higher level and rank will beat the crap out of you, and sometimes its impossible to stop it.