CTP is NOT an expansion, it is a game made by a different company.
Civ 3 is a POS for many, many reasons. Far too many to list in completeness. Here are some: First off, they specifically cut BASIC features from Civ 2 (multiplayer, organization of data files, moddability, victory points/conditions, etc.) so that they could release 2 expansions "featuring" them. The fact that these features came about in Civ 2 expansions is irrelevant because they were new features at the time. It is completely inexcusable to NOT have these in the original Civ 3. The "expansions" for Civ 3 are worth about 5 dollars in new features (and that is being generous). Next, you have the fact that modding Civ 3 for years took hacking of the game files in order to work. The Scenario Editor was a complete POS. Now it is just a POS. It took many patches to get it to even be useable to mod the game! Next, they removed the text files for storing data!!! This was a HORRIBLE move. I think we can all agree that text files are the way to go for moddability. Furthermore, they nerfed espionage/sabotage. It is damned near impossible to do it more than a few times in Civ 3 unless you are so much larger and more powerful than the other empires that you can afford it. But by that time, it will not make a difference! Additionally, the AI cheats much more horrendously (especially in the diplomacy department) than in CIv 2. The AI empires regularly trade their technologies for ludicrously cheap amounts. Next, they removed the EVENTS system! Especially in ToT, it had so much potential for creating cool mods and scenarios! Furthermore, you have the VERY common experience of ridiculous combat results like a tank being defeated by PIKEMEN! (who are ONLY armed with pikes, maybe some swords; any explanations of "they have mines and such" are ludicrous apologetic justification for garbage game mechanics) At least you don't have the classic Civ situation of a warrior defeating a battleship!
Additionally, they removed EVERYTHING good added in the ToT expansion. There were dozens of cool new abilities for units you could use. All are gone. The only thing they kept was the ability to assign settler abilities to units individually instead of all at once.
I would continue, but I must get to class now. Expect more in the near future!