You don't really need to worry about your health or safety when you come to Delft. This is a city, but it is not a huge city, and you don't really need to worry about criminals or other seedy characters like you would in Amsterdam. This is a nice town that encourages having a good time, but also wants you to respect the laws and obey the authorities. With that being said, if you are a bit rowdy or disrespectful, the locals will try to tell you to stop. If you don't, the authorities will come down and try to ration with you. However, if you continue to be a nuissance, you will have the legal system thrown at you pretty harshly. You shouldn't really be finding yourself in these types of positions in the first place, but it's is something to keep in mind if you plan on going out and drinking. You should always watch out for pick pockets, but you shouldn't expect too many of them in Delft anyway. Always keep your passport on you, no matter what. This can be the difference between a problem and a minor inconvenience.