This section of the docs is about Urbit's Nock interpreter and runtime system Vere, which is written in C. This is of interest if you're planning to work on the Urbit interpreter, you're a language implementation geek, or you don't really understand anything until you've seen the actual structs.
Developer Docs
U3 Overview
An overview of the noun-wrangling part of the runtime, U3.
Conn.c Guide
Using `conn.c` to interact with a running ship from the outside.
How to Write a Jet
A jetting guide by for new Urbit developers.
C3: C in Urbit
Under u3 is the simple c3 layer, which is just how we write C in Urbit.
U3: Land of Nouns
The division between c3 and u3 is that you could theoretically imagine using c3 as just a generic C environment. Anything to do with nouns is in u3.
U3: API Overview
A walkthrough of each of the u3 modules.
Cryptography
References on the cryptography libraries utilized by jets.