As ashlar is a common stone facing for castles, an understanding of it is important to good modeling. One of the easiest ways of thinking about ashlar is comparing it to concrete Allan Blocks, used in modern landscaping and found in nearly every DIY store like Home Depot, or full-service landscape supply.

It is important to remember that ashlar was only a facing! Here is a description of the building of castle walls:

The mortar used to bind the stones together was a mixture of water, sand, and lime. The inner and outer faces of each wall were constructed first. The stones were carefully fitted and mortared in horizontal layers called courses. When a height of three or four feet was reached, the space between these two narrow walls was completely filled with rubble - a mixture of stones and mortar." - Excerpted from Castle by David McCaulay (Houghton Mifflin, 1977).

Definition of ashlar from Castles on the Web: Ashlar - Squared blocks of smooth stone neatly trimmed to shape. Read another definition here.

According to Funk & Wagnall's Dictionary (1980): 1. In masonry, a roughhewn block of stone. 2. A thin, dressed, squared stone, used for facing a wall. 3. Masonry made of such stones. Also ash' ler. (ORIGINS: aiseler, ult. ,L axilla, dim. of axis board, plank)

Ashlar's strength lies in its dimensions: The stones for a single project need not be of equal proportions, but the base of each stone should be wider than its face, and the stone should be relatively square. The finish of the facing depends on two factors: The type of stone, and the quality of the workmanship (also related to the cost factor - how much does the owner/builder want to spend?)

These examples of ashlar were taken from The Stonework "How-To" Site .

Relevant Quotes:

From "The Mediaeval Builder and his Methods" (Andrews 1922, Dover reprint 1999):

"The masonry [before the 15th century] had thick mortar joints, and stone ashlar-work of but moderate quality... Ashlar face-work was built up of worked 'acheleys' or squared stones sometimes called 'essicis (Fr. 'assise') and had a proportion of 'thrughe stanes' or bonders at intervals; these extended right through the wall thickness... The in-fillings of the wall within the ashlar casing was mape up with rough stones called 'raggs' or 'royboyll.'

According to Sweetman (1999), ashalr work replaced pre-Anglo-Norman "uncoursed drywall walling of large boulders and blocks..."

Charlotte-Mecklenberg Historic Landmarks Commission's page on stonework clearly shows the different stone wall types with black and white drawings of random rubble, coursed rubble, mosaic, random and coursed ashlar. They also have some basic but helpful architectural info here. (Thanks to Harliquinn Whiteshadow for this link!)

Coronado also has excellent pictures showing different patterns of both ashlar and rubble.

Astley, to the left, is an example of ashlar, roughhewn stone blocks, of various sizes, forming two faces between which there is a thick in-fill of rubble. The quoins, or corner stones, are hard to see (left of picture) but they are larger and squarer than the other stones. Compare Astley to Baltersan (right), which is ashlar intermixed with rubble, no doubt for economy's sake or due to a paucity of large stones in the area. Note, though, that the window casing is made from accurately hewn stone, and that the sill and lintel are single pieces of stone. A castle like Baltersan may once have been "stuccoed" in limestone, hiding the stonework and rendering the building a gleaming white edifice. This was not an uncommon practice.

Tolquhon Gate is another good example of ashlar stone work. The stones are not of equal size or dimension, but there are few small stones. Note the more carefully cut quoins and arch stones for the archway, no doubt dressed by a master mason or trusted apprentices at a much higher cost than unevenly sized ashlar or coursed rubble.

A building from Slains Castle...

Some links that may be of interest:

Restoration of the stonework at Ballytarsna Castle. Click on the Update link at the bottom for an interesting write-up.

Building a Castle- No pix, but a great source of information.

Bontool has line drawings that may be helpful to the stone modeler.

The Heritage Trust of Lincolnshire has an excellent page on Navenby's limestone, and encompasses far more than just ashlar.

Farrar's Natural Stone website has downloadable CAD files for ashlar and other stone/cement pavers.