Restricted Areas

You can't just walk everywhere in a city like Bogota. Some areas are to be avoided at all costs. Unfortunately there are no signs warning you when you are entering (or leaving) such an area. The city is structured just like an American / designed city. You have Calles (Streets) and Carrerras (Avenues). A helpfull person told me 17th,16th,15th,14th Avenue: Dodgy; 13th and lower: Don't go there... Great ! My hostel is in the 15th, so from 7 pm I stay inside and even during the day, the door is locked most of the time. It didn't look that dangerous to me though...
The second day I was there, I was attracted by a building on top of a nearby hill. The climb was so steep that they constructed a cable-lift with two cabins that hang almost vertically in the air. A stunning sight. I started to climb a couple of stairs but only after 10 m up, I was out of air, which surprised me since I've done may tracks before. Later I realized it must be lack of oxygen because of the altitude of the city. I was about to turn back when two guys walked past me and one said "Monserrate ?" which I figured, was the building on top of the hill. So, I anwered "¿Si, es lejos ?". They said "No !" so I started following them. Only after 10 meters, suddenly the guy in front of me turned around and said "Give me your money, mottafukka !" while they other one pointed a knife at me (he held it in a strange way though, with two of his fingers between the blade and its sheeting, so that, when it would fold, he would cut himself). So I took my wallet, first took out my bankcard and put in the pocket of my trousers and then handed over about 30.000 pesos. In the mean time the other guy took my backpack of me (I was so stupid to carry that around) which contained my laptop, my camera/phone/mp3 player/flashlight/alarm clock and my passport. The one guy had my backpack lying at his feet so I jumped forward and tried to grab my backpack in an attempt to run off with it, but I missed. Seeing that I was going to make trouble the guys started backing off and I started following them shouting to give my backpack back (I hoped to alarm other people and the police). The person with the knife threatened to throw stones at me, which he eventually did since I didn't let go of them. One heavy stone hit me in the knee and the other hit me at my ankle. Some people that were living there were watching the scene and I asked on of them what to do (he was an older guy and not likely to interfere). He told me to run down to get the police which I immediately did but by the time we came back, there was nog sign of them anymore, of course.
After searching for 1,5 hours, the police gave up and I went back to the hostel. A few hours later I started feeling dizzy in my head (I had a couple of scars from the knife) and I was afraid I might have gotten an infection from the knife. Apparently I also strained some muscle from my knee up to halfway my back, so I was limping like an old man through the hostel.
The owner arranged a friend of hers to go with me to the hostel where they desinfected the wounds and said everything else was ok (but why was I so stupid ?)...