For each level you are asked to write a smart contract based on the given interface. You are free to write the solution as you see fit as long as your contract adheres to the contract interface. In practice this means, for most levels, you will be given an empty function and asked to write the logic. Once you have written your solution you will need to deploy the contract and submit the address to receive a score.
You can submit as many times as you like. The contract saves your max score with the lowest gas.