(written from a Production point of view)
When the Cardassians were introduced in the fourth season episode "The Wounded" of Star Trek: The Next Generation, they were represented as a major Alpha Quadrant species with a major starship to match. Simply introduced in he script as a "CARDASSIAN WARSHIP", the design of which left to the imagination of the series' main production designer, Rick Sternbach, who had to come up with a starship design for a major Alpha Quadrant power. This Sternbach did, and his design, the Cardassian's major Galor-class starship, eventually became the basis for an even more advanced Cardassian vessel, the Keldon-class. Designed surreptitiously by the Obsidian Order for a planned joint attack on the Founders with the Tal Shiar ("The Die is Cast"), it was far more powerful than the Galor-class it was based on.
The studio model of the Galor-class was eventually designed by Sternbach, with assistance from Peter Lauritson. The inspiration for this model was the starting shape of the Egyptian ankh; in fact, according to Sternbach, the physical model has "a little temple and some fire pots and little tiny pyramids for the phasers." Sternbach commented further on his design:
"The orthos furnished to VFX for Ed to go by were fairly simple pencil affairs showing the basic dimensions/masses. Ed also had the top and bottom 3/4 views to use for detailing. I did some more detailed ortho vellums before and after the miniature was done(...). (X) One of the really early Galor designs had the ship looking more like a scorpion, with a few dorsal tail pods and some forward cannons looking like pincers, but it felt a bit too blah. The pods disappeared along with the cannons, but a flat disruptor forked tail thing remained. Once the basic shape felt right, the detailing began, in the Egyptian ankh mode. (X) The tail (...) was a big aft-facing disruptor weapon similar to the forward-facing one on the Klingon Attack Cruiser. The Cardassians seem to like borrowing other races' technology. The big square under the bridge section was really meant to be a nav deflector, but the VFX folks shot beams out of it. The green pyramids on the "wings" were smaller disruptors like Starfleet's phaser strips." (X)Sternbach eventually sold his design concept sketches as Lot 277 in the 2003 Profiles in History The Ultimate Sci-Fi Auction for US$750.00.
The actual, physical studio model was originally built by Ed Miarecki and Tom Hudson of Science Fiction Modelmaking Associates (SFMA), receiving the order on 14 November 1990 and delivering the finished model on 5 December 1990. They were awarded the commission, as the franchise's regular model vendor at that time, Gregory Jein, Inc., already had their hands full in constructing the full-fledged Nebula-class model, slated for a likewise début in the same episode. According to Miarecki, "This miniature is about 37 inches long, and was built in 2½ weeks, by Tom Hudson and myself, (our initials, "TH" and "EM" are built into the detailing). Originally contracted for the ST-TNG episode "The Wounded" it has been seen in many episodes of ST-TNG, ST-DS9, and in the pilot of ST-VOY. It has the distinction of being the only ST-TNG spaceship miniature built on the east coast of the U.S. and was one of the last few TV filming miniatures built for "Star Trek", before the conversion to all-CGI spaceships."  Hudson, on his website, has made the following observations on constructing the model:
"In 1990, my modelling partner in SFMA, Ed Miarecki, put in a bid to build a starship model for Star Trek: The Next Generation. For several years, we had been making various props for the show, including tricorders and medical scanners, and we had been building various science fiction models for years, so we thought we'd see if we could build a filming model for Trek. We heard they were going to need a model built for their new race of bad guys, the Cardassians, so we put in the bid. One day not long after, the phone rang, and it was Ed. "We got the job!" he yelled. It was like, "All right! We got the job, we got the job!!" We had three weeks – And the realization was hitting us that the clock was ticking. "Oh my god – we got the job..."The deflector has been frequently and erroneously colored blue on promotional images, despite appearing only yellow or red on screen. The model debuted in "The Wounded" and was last shown as stock footage in DS9: "Favor the Bold" and VOY: "The Voyager Conspiracy".
"Star Trek art department gurus Rick Sternbach and Mike Okuda sent out the plans for the ship, along with color chips showing what colors they'd like the ship to have. We then started looking at how were were going to build the thing. The next week was a whirlwind of activity as we started getting ready for the project. I started cutting the aluminum framework that would be the interior structure of the ship in Kansas City, and Ed got busy in Massachusetts with other aspects of the build – Getting materials and lining up special services we'd need, like a big vacu-former and custom neon fabrication. After about a week of prep, I flew up to Massachusetts and Ed and I dived into two solid weeks of 16-hour days, building a starship. It was probably the most fun two weeks of my life. The project went largely without a hitch, even though it was the most complex thing either of us had ever built." 
Becoming the Keldon-classEdit
The studio model was later modified by Tony Meininger at Brazil-Fabrication & Design to include the addition of fins located on the aft end of the hull, and a large hull module attached to the upper-middle section of the hull, creating the Keldon-class, typically portraying a starship class belonging to the Obsidian Order. It made appearances in "Defiant" and "The Die is Cast". The additions were not permanently affixed, so that the model could be easily reverted back to its original configuration. Unlike its progenitor the Keldon-class was never converted into a CGI model.
The Galor-class model (measuring 36" × 18"), in its original form, was listed in the 40 Years of Star Trek: The Collection auction as Lot 708 with an estimated sale price of $3,000-$5,000; it sold for $20,000 ($24,000 with premium) on 7 October 2006. Several Galor-class camera test models were also sold off on various IAW auctions in 2008.   
For the last two seasons of Star Trek: Deep Space Nine, the model made the transition into the digital realm. The physical model was sent to Foundation Imaging as a reference for Brandon MacDougall, who built and mapped the CGI model, making inspired use of a flat-bed scanner. MacDougall recalled:
"I set the the motion control model of the Cardassian Galor Cruiser on my desktop and started to input measurements into our 3D software. First the hull of the spacecraft and then the primary disk and finally the outside panels or what my boss lovingly calls "Nurnies". Next it was time to paint the 3D textures for the 3D model of the Cardassian Galor Cruiser. One of my fellow 3D modelers, Koji Kuramura, stopped by my office and pointed out that if I put the motion control model of the Cardassian Galor Cruiser on our flat bed scanner I could get a good color scan and panel detail. One thing I have learned over the years working with concept designers like Syd Mead and Jim Martin, and now with Ron Thornton, Paul Bryant and Rick Sternbach is to keep a very open mind! The scanner approach worked very well for the base 3D texture and with little painting in Photoshop I was finished." (Sci-Fi & Fantasy Models, issue 32, p. 54)The CGI-model made its début in the Deep Space Nine sixth season epidode "Sacrifice of Angels". (X) It has never been featured as the Keldon-class.
Yet, for all MacDougall's acumen and his model's appearances in the Star Trek: Ships of the Line calendars and their book derivatives, Publication Editor Ben Robinson found the model wanting in detail and had Fabio Passaro re-render the model for publication in its 2014 outing of the Star Trek: The Official Starships Collection.