The Master Player's Book provides statistics for siege equipment and fortifications, as well as a system for handling sieges, the "Siege Machine" (based on the "War Machine").
Seige equipment, Master Players' book (pg. 25)
Sieges in D&D owe their lineage to the Siege of Bodenburg, a medieval miniatures wargame, run at the first Gen Con in 1968
There were 2-3 pages devoted to sieges in the Chainmail rules by Gygax and Perren, which were themselves inspired by the Siege of Bodenburg.
The section on "Construction and Siege" in the AD&D 1e Dungeon Masters Guide (pgs. 106-110) also covers siege equipment and rules.
I've always thought it would be fun to run a siege involving the Keep on the Borderlands using one of these systems, although have never gotten around to it.
