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Levels in vanilla Doom are limited in size by the blocklist, which essentially enumerates which linedefs cross which blocks in the blockmap. This list may only contain 65535 entries, including "begin" and "end" entries for each block. In principle, therefore, a level could contain as many as 32407 blocks (see figure), but for real maps the empirical limit seems to be somewhere between 10000 and 14400 blocks; the more complex the map, the smaller its area must be.
- The Unofficial Doom Specs v1.666, section 4-11