Old Spawn can either mean the location in RPS Nova where the original spawn point for the map used to be, or the actual building where the spawn point was housed. Either way, it no longer exists, having been replaced with New Spawn (hence the name) which was moved out of RPS Nova to avoid the lag problems caused by everyone spawning in the most densely constructed part of the entire map.
The original spawn was built by DRoseDARs (confirm?), and was designed purely with safety in mind. It was made of obsidian and was surrounded by cactii to provide a high level of security from mobs. Inside, the four compass points were marked with signs, and more signs detailed the server rules. The door was originally controlled by a wooden plate, but given how laggy RPS Nova was, this caused problems and people often found it almost impossible to actually pass the doorway!
Rustyshakelford created an exact replica of Old Spawn, which can be found behind his house just to the north of where it originally stood. Eltagno took the opportunity to replace the original site with a Memorial Garden, from which one can still peruse many of the earliest and most instantly-recognisable sites of the server, such as the Clock Tower, the Snowman, and n00b Castle, amongst others.