From the Beneath & Beyond Blog:
Tower of Mrannor is 93 pages of maps, keys, background and monster descriptions. It details the three levels of the tower itself, and four dungeon levels beneath (plus a sub level). For the fifth level you pass through an Interdimensional Gate into a huge wilderness area containing various separate adventuring locations, some of which themselves are multi-level. Further it contains gates into four planar adventures.
Mrannor is firmly rooted in the tradition of megadungeons: in history (it was created by an Evil Wizard), in location (it lies underneath the Wizard’s tower on an island), and in scope (256 rooms across 24 levels or areas spread over 12 pages of maps). But it is also definitely a megadungeon of the late 70s – it is not the minimalist monster + treasure style of the earliest examples. Of the two main strands in late 70s dungeon design, naturalistic and puzzle, it fits the naturalistic mould
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