The Bergfried is the tallest and strongest tower of a Germanic castle. It is about 20 - 30 metres tall and 8-10 metres wide. The entrance was usually placed 8 metres above ground level, they only had a very sparse amount of windows and arrow slits were also rare. These towers usually carried a roof and were a symbol for wealth and power.
I placed the entrance a few metres above ground level. The "Bergfried" is only accessible from the lower hoarding, which connects the wall and the gate.
There is only one window; in fact most of these towers had hardly had any openings in their thick walls, as they were meant to protect the castle from incoming projectiles. Any opening would weaken its structure, thus limiting their active role in warfare.
Usually, there were no siege weapons placed on top of the tower (huge siege engines were indeed a rare sight on most European castles, as there was simply not enough space, this was different on Crusader castles). Instead they carried a simple roof, as it was very difficult to seal a flat roof properly from incoming rain.
Hoardings, which allowed a protection of the ground beneath, surround the top of the tower. In the past defenders used arrows or boulders to engage the enemy. By the way, there is no evidence that hot oil or bitumen was used to be poured down on the attackers.
Many people believe that the Bergfried was the place of last retreat. That is only partially true, as it would not make sense, if aid was not on its way. Today many scientists believe that the function of a "Bergfried" was rather symbolic and protective.
I have added some real examles
Linn castle is one of my favourite castles near Krefeld. It is extremely well preserved and except for the new roof it is a truly medieval castle. The old entrance to the "Bergfried" is a few metres above ground level - look for the two corbels on the left, which mark its position. It also contains a very interesting toilet and fireplace.
Neublankenheim has got a very small "Bergfried". It is a minor, but interesting castle in the Eifel. The front is protected by a "shielding wall" which were common in South-West Germany. Sometimes these walls replaced the "Bergfried". In these cases they were very strong and high. A very well preserved example can be found at Berneck castle.
The two Manderscheid castles are fantastic. Their sight is truly breathtaking. You can visit the Oberburg for free, while you have to pay for the far bigger Niederburg. The Bergried is rhombic. That makes it quite unique. You can even see its original (yellowish) render. In fact many castles were not stony grey. They had a white or yellowish render to make it clearly visible in order to show power and strength. You can also see the toilet, which is inside the wall itself. In late medieval times small round towers were placed on top. I used a similar design for the "Wohnturm" of my MOC.
Winneburg, a true beauty near Cochem/Mosel. If you ever visit Cochem, you must visit the castle. Again you can see that the entrance is several metres above ground level.
The Wildenburg, near my native village. This is a true beauty. Before it was demolished, it was a even more impressive. The original entrance can still be seen. A wooden plank reached from a neighbouring building right to the arch.
This is the "Bergfried" of the Godesburg in Bonn. It carries two rows of corbels, which carried the hoardings. Just above the first row of corbels, you can see its original entrance. Although, the castle has been spoilt by concrete building in the early 1960s, the tower is worth a look.