It would have taken longer and been less efficient to build from scratch, and so in many cases he chose to build on existing fortifications. After all, you don't build a castle just anywhere: you need it to have resources, such as fresh water. Towns were established the same way; no one built them in a desert or on an inaccessible peak (unless you built them for other reasons than comfortable living). So
Since many towns already had defensive walls and the resources needed for living, many of the new structures were set up there. The castle would take up space, however, and the town was already being used. The solution was to demolish local houses and buildings.
Records of castle-building hint that, for instance, 166 houses in Lincoln were destroyed to build its castle. In Norwich, 113 families had to be displaced. Cambridge got lucky: only 27 houses were destroyed to make way for the new castle.
More motte-and-bailey defenses were set up in the west than the east, likely because the east was relatively settled, but in the west there were likely problems coming with Wales (and they would be coming for a long time). In all, at least 741 motte-and-bailey castles were built in England and Wales over the next few generations. When the Bayeaux Tapestry was made, it even included a fighting scene taking place at such a structure; in the photo above you can see a wooden palisade on a hill with soldiers fighting at the base.
I know that motte-and-bailey sounds like a primitive style of castle,—it is certainly no Neuschwanstein—but it was just the first stage for greater things. Next we'll look at a motte-and-bailey that everyone's heard of: Windsor Castle.