First of all, bank robbery is a very old concept. Some of the stuff used in the levels back then is no longer necessary. For example, the original BR levels had built-in time limits using complex trigger systems. This sort of thing is now obsolete.

The only thing your level needs is a way for points to be scored using coins. You can create this by copying another level, quite easily. The second thing that is needed is to seperate the attackers and defenders: for example, defenders should not be allowed to enter the attackers base. This is also easily done with very basic triggers. Once you manage this, you will essencially have a working BR level. Don't worry about more complex things: to switch the attackers and defenders, all that you need to do is make two versions of the level, one of them with the start positions swapped. Both levels hosted one after the other will constitute a single match. Nice and easy, and I strongly advise you use this method over the more advanced one seen in older levels.