Castles were usually built on high ground and a river is very seldom on the highest point in an area. The erosion to the castle base would make it a poor spot. A castle needs to be strong at all points.
because boats could easily come in and tear down the castle
Windsor Castle is near the River Thames.
Due to the lack of roads, it was easier to supply a castle near the sea by boat, than to supply an inland castle by horse-drawn cart or pony-train. Also, a castle by the sea could guard (control) shipping entering or leaving a river's mouth.
Build it near a river.
The normans wouldn't have placed it near another motte and bailey castle in case of constant raids for land, They should have it near a water source such as a river or lake The normans should build on high ground so they can see if someone is going to attack and it is harder for the enemies to attack hope you find this answer helpful
There are five castles on or close to the River Thames; Hadleigh Castle, Essex Tilbury Fort, Essex. Tower of London Windsor Castle, Berkshire. Wallingford Castle, Oxfordshire
Because, Tutbury castle, is built on a hill, so it makes it hard for the attacker, that is coming for instance war, a trouble, as the wall are immensely high, its impossible to climb, also they can see there attackers from up in Tutbury for miles because of the height, and there is a river near by called the River Dove, this is a river attackers have to cross to get any were near the castle, so the more difficulty it is to reach the castle for war :).
for water resoures and for god soil for cultavitation
because they can use it for bathing,drinking and laundry
it is quicker and easier to build the castle.
Where it is easy defended. Near a river, some woods, or on a hill, for instance, where the castle-dwellers will have all the advantages when confronted by enemies, but also when it comes to collecting resources like wood and food for the residents to survive.
the merchants built settlements outside of the castle is because they could catch oncoming travelers and make more of a profit