[kbp]

> Some checking also happens at compile time, even more if you intentionally include type declarations. (Type declarations are part of the ANSI Common Lisp standard.)

Just a note that this is entirely implementation-dependent. SBCL, for instance, is very good about using type declarations as correctness checks (those that can't be statically verified transparently degrade to runtime assertions) but their exact behaviour isn't specified in the standard; for example, implementations are free to take them as declarations that the programmer knows things the implementation doesn't and to trust them, which could cause weird bugs if they're not correct.