Crusader Rex is in the family of Columbia Games titles related to its very popular Hammer of the Scots. There are enough significant differences between the two that you couldn't call them sister games or part of a system, but they're at least cousins.
This is hardly surprising, as lead CR designer Jerry Taylor also designed HOTS. It's not a straight copy however. While both games are set in the Middle Ages, there are important differences in geography, culture and the historical situation that result in a very different game environment. Whee the games are most similar is in basic game procedures for movement and combat.
Crusader Rex is a block wargame, which means it uses wooden blocks to represent the combat units. The blocks are usually on the map placed on edge, so the owning player can see the current strength while the opposing player simply sees a block, Stratego-style. This provides a lot of scope for bluff and fog of war, especially because the strength of the opposing pieces can vary from 1 to 4 in combat value, depending upon which edge of the block is uppermost.
This elegant combination of fog-of-war and step reduction is the distinguishing feature of the block wargame genre and Crusader Rex makes good use of it. The combat ability of the block can vary considerably and both armies have much more territory to cover than troops available t do it properly, so there's a lot of scope for strategy.
The engine driving the game turn are event and move cards. Each turn six are dealt to every player and players use those cards to order moves by 1-3 groups of units from point to point. When friendly and opposing units occupy the same place there's a battle (or sometimes a siege) that involves up to four rounds of combat where units battle against each other.
There are significant differences between the rules in their currents state and the rules originally included with the game. Columbia Games is a big believer in "living rules" and the most up-to-date version of the rules for every game is available online as a free download. While overall a benefit to the player, one sometimes wonders if game companies using this policy aren't tempted to skimp a bit on the playtesting, figuring they can always fix it later.
In the case of Crusader Rex there have been major changes in the rules, especially as they relate to Saracen deployment. For both sides the majority of units are now permanently eliminated from the game when they lose their last step, which is a change from before when only select elite units were gone for good. The number of "nomad" Saracen units drawn at start has also been changed-- now 4, before it was 6.
These adjustments seems to have solved some early play balance problems, so they are welcome, although it would have been better to have discovered it before.
These changes also suggest how tenuous many design decisions may be in wargames. Many key game elements come about purely for gameplay reasons, not because of history. There may be good historical reasons to make units permanently eliminated or subject to being drawn again, but evidently they didn't come into play here. History-minded players should keep in mind the compromises that game play may require before getting too dogm,atic about what's "realistic" in a wargame and what is not.