I ran across the Dragon Crest playset at toyranch’s Flickr. I found nearly the same set, renamed Mysterious Castle, in the 1982 Sears Christmas Catalog. The date on the Dragon Crest set is a guess. At some point in 1979, Multiple Toymakers ceased operations and Miner Industries partnered with Union Major. Many or all of Multiple’s molds fell to Miner, much like Mego acquired various Marx Toys’ molds in ’79. It was the year that marked the beginning of the end for the traditional playset.
The big addition to Mysterious Castle is the cheesy dragon and his/her lair. The mat is also a little different. Beyond that, it looks like all the pieces have been recycled from previous Multiple sets, including Castle Attack and Castle Warfare (1964), Battle of the Knights (1965), and Carry Case Castle (1969). The “fiend” figures are from the famous Pop-Top Horrors designs first produced in 1964.
Marx started putting out medieval sets in the ’50s, but Dragon Crest marks the first time the traditional knights/castle theme was combined with monsters, including dragons, thanks to the booming fantasy/D&D market.
UPDATE (4/6/14): I’m now betting that DFC’s Dragonriders of the Styx Fantasy Playset (1981) was the first to present a theme inspired by D&D. That would mean Dragon Crest was released at some point after Mysterious Castle. I’m changing the date on the set from 1981 to 1983 for now.
2 Warps to Neptune 
Cheesy? No, that dragon is awesome. But a green dragon in a desert? Really? Now we know why they went out of business.
Dragons don’t need water, only treasure.
Back in 1964-1965, these monsters were called “dungion demons” Sold in bag sets, as well as giveaways with a six pack of fritos corn chips.
Multiple Products Corp started up about 1954 with a Canadian subsidiary Multiple Toy Makers. Eventually the Multiple Toy Makers name was used on US product as well. In 1965 they sold out to the Loral Corporation who was in the space industry and decided to branch out. Product was marketed as MPC a division of Loral Industries and the Multiple Toy Makers name might also be on the package. In 1968 realizing they were out of their element Loral sold MPC to Miner Industries. Packaging would now say MPC a Division of Miner Industries and the Multiple Toy makers name might also be on the package. In 1979 Miner filed for bankruptcy emerging with new partner Toy Major (or Union Major). The Multiple Products Corp. & Multiple Toy Makers names were no longer used (perhaps a bankruptcy condition). Some packaging might have one or both names Miner Industries and or Toy Major. Some packaging had nothing. They went out of business about 1985 and American Plastic run by Jay Horowitz bought many of their molds (he also bought many of the exMarx molds) and started ReMarx which lasted into the mid 1990s.
Loral sold MPC in 1967 not 1968
Miner Industries who owned MPC (Bought in 1967 from the Loral Corp. who had bought MPC in 1965) filed for bankruptcy in 1979. They emerged from bankruptcy with a new partner Toy Major and the Multiple Toys name was dropped from all packaging (court order as part of the bankruptcy??). Packing could say either Miner Indutries or Toy Major or both names or no names at all. They made a Mysterious Castle playset in 1980 or 81 with the bright green monsters. In 1982 they added a cheesy cardboard dragon and renamed the set Dragon Quest selling it in the Sears 1982 Christmas Wishbook. Miner seems to have gone under for good about 1985/86. The location of the monster m old is unknown and no reissues have been made. The leftover bright green mobsters were found by John Stein.