Tip: When determining a time limit, remember that players won't be as familiar with the level as you are. If you normally complete the level with around 100s left on the timer, others might run out of time on their first try.
The level number of an overworld tile changes after a particular RAM address has been set. This can be done for any number of levels, and each can change depending on a different address. Check out the readme and the asm files for more info.