The multiclassing system is one of the best things about Castles & Crusades (C&C). Once you read this and understand the system, wow, this game is like very few others in the options and flexibility the game gives you. Not only in what you can combine but in the multiple ways you can combine them.
Many B/X games give you a class or a hybrid, and that is that. C&C gives you a box of types, treats them like Legos, and invites you to build the character you dream of playing. This isn't a GURPS-style point-buy system as we are still working with traditional classes, but being able to pick 2 or 3 and combine them in various ways gives me hundreds of ideas of unique characters I just can't get in other games quickly - or at all.
The Players' Handbook has 13 classes, while the Player Archive ups that number to 36, already an incredible number of standalone options, and this is more than a lot of B/X games deliver. And then the game turns around and invites you to combine them, allowing you to wonderfully simulate the hybrid martial-spellcaster classes of AD&D.
You can multi-class, which means you combine the XP-per-level (plus a little) of two to three classes and advance in them simultaneously. You can use class-and-a-half. Which gives you your primary class plus a half-level supporting class, perfect for the AD&D style paladins or rangers where they have a primary martial focus plus a slower-advancing supportive spellcasting class. The Player Archive adds more options, such as class-plus, dual-class, and reclassing.
Add this to a system that unifies all skills, feats, class ability, and saving throws under a unified ability-based system (SIEGE Engine). You have a more straightforward design than many B/X offerings, with less to write down and track on character sheets. I don't have to write down and reference arbitrary saving throws and attack matrixes. Everything is ability-based, which is instantly familiar to 5E players.
I had a few issues with the SIEGE Engine at first. Still, when I realized the system's class-ability and roleplaying uses, I started to appreciate the genius of what the designers did. Want to invent a new ranger class ability, such as Tree Ring Reading? Just develop an idea, talk it over with the GM, and tie it to an ability check based on your class level. You are freed from searching through and referencing endless lists of feats and skills, your powers are more situation-based, and the core system stays simple and clean.
This is a best-of-class OSR-style game that does a lot of what I love, keeps things simple like B/X, and invites you to get in and tweak to your heart's content.