In addition to Desdemona's post about the fore- and sterncastle I have to say that, as far as I know, "Both a fore- and stern-castle is a structure at the bow of a ship" is not entirely correct.

The fore-castle relates to Medieval shipbuilding where Ships of War were usually equipped with a tall, multi-deck castle-like structure in the bow of the ship, from which archers (and later musketeers) could fire into an enemy ship. It could also be used as a defensive structure if the ship was boarded.

As shown on the Bayeaux Tapestry (invasion of England in 1066) the Norman longships have fortified platforms for archers at each end. They resemble small castles, and the notion of a castle from which to fight on shipboard becomes part of naval terminology.

A similar, but usually much larger, structure was at the aft (back) end of the ship, often stretching all the way from the main mast to the stern, thus called the 'stern-castle'.

Having such tall upper works on the ship of course influenced sailing performances. As cannons were introduced and gunfire replaced boarding as the primary means of naval combat during the 16th century, the medieval forecastle was no longer needed, and later ships, like the Galleon, had only a low, one-deck high forecastle.

Some sailing ships and many modern (non-sail) ships have no forecastle as such at all, but the name is still being used to indicate the foremost part of the upper deck (although often called the foredeck) and for any crews quarters in the bow of the ship, even if below the main deck. Related to the crews quarters is the phrase "before the mast" which describes anything related to ordinary sailors (as opposed to a ship's officers). The forecastle is often referred/reduced to as fo'c'sle. I used to think they were talking about a 'foxhole' but this makes sense *G*

The stern is the rear or aft part of a ship, technically defined as the area built up over the sternpost, extending upwards from the counter to the taffrail.

The stern area has always been the location of the steering equipment (rudder, tiller, ship's wheel, etc), and by extension became the domain of the ship's captain and other officers. In particular, the stern was the location of the officers' quarters and during the Age of Sail it became the most opulent part of the ship, with rows of windows, galleries, walkways and elaborate decorations. This resulted in a certain amount of vulnerability and the goal of much maneuvering in battle was to achieve the "stern rake", in which a ship would pour its entire broadside into the stern.

In the early 19th century, the stern of larger ships became gradually more rounded, and with the advent of screw-powered vessels, the stern became the location of this equipment, the officers moving elsewhere, although British ships still contained an Admiral's sternwalk until well into the 20th century.