posted 03-24-2001 12:59 PM
It's not just castles that were built alongside rivers and water-courses - most towns are constructed in these places too.
Water is vital for living. You have to remember that there were not usually public water supplies to bring water to towns, or castles, from other areas. It was only when places became settled and prosperous that public water supplies were installed (usually paid by subscription).
From a defensive perspective, I agree with what Merlin has pointed out.
One other reason that castles were built alongside water-courses is that they were often constructed on the site of previous stongholds and fortifications (which presumably built in such places for the same reason as castles).
As an example, many Roman forts in Scotland were built at river confluences, which provided double protection. If these sites provided a vantage point, so much the better. Alongside rivers such as the Clyde, there are a number of sites where Roman forts were built alongside river confluences and also provided vantage points to Lochs and inlets on the otherside of the river. Once the original builders of these sites were defeated, it was common to build your new castle on the same site - for one thing, you don't want your old enemy living next door (you've got to show who's the new boss) and it makes perfect sense to reuse as many of the materials as possible - saves a fortune in quarrying effort.
The other thing about a river confluence is that it is very much like a road junction, an ideal place to put shops and trading posts. Build a castle in these sorts of places made it easy to collect taxes on any trade taking place (castles were often an important part of a nations government,legal and taxation systems). There are lots of castles in England that are at river confluences AND road junctions so it's no small wonder that these are often surrounded by towns that serve and enjoy the protection of such castles.