i can't figure out how to withdraw money from bank to buy towers

You can't... You must use the mana that you acquire in chests and the mana that monsters drop, everything in your bank cant be "summoned" to you in between waves to upgrade/repair/build.

You can't .. Once the waves finish and you 'bank' your mana, the only place you can spend it is in the tavern to buy from the shop, player shops or upgrade weps/armor with it

OOO, makes it a little tricky then...

It's not tricky, it's a tower defense game which mean at its heart it's a Strategy game.
If you could use banked mana to build, the game would be way too easy. No strategy in that.