Catonsville, Md., was crawling last weekend with Dungeon masters, druids, grown men dressed in mail and tunics and carrying double-bladed axes, teen-agers with sunken cheeks and paranoid eyes wearing Vietnam battle fatigues, and lots of normal-looking folk, many in shorts to evade the heat--all of whom regularly retreat into the fantastic and imaginative world of gaming.

The warriors took over the entire Baltimore campus of the University of Maryland last weekend for "Origins '82," the eighth annual "National Adventure Gaming Show." In classrooms, on tennis courts and on grassy lawns battles of all sorts were waged by hobbyists who amuse themselves by playing at war like others play chess or Scrabble.

The gathering brought together at least three distinct groups of gamers--the historical re-creationists, the war gamers (who use conventional-looking boards and pieces) and the fantasy gamers (the Dungeons and Dragons crowd belongs to this last group)--for what has become an annual ritual. More than 2,500 people from all over the nation converged on Catonsville for three days of gaming (everything from a reenactment of the Battle of Waterloo on historically correct terrain with 5,000 tiny, painted metal soldiers to "Psionic wars," a role-playing game set in the future in which psychic power is the weapon of choice), seminars ("Methodical Mayhem," "Soviet Military & Industrial Decision Making," "Castle Design and Siege Warfare"), and spending (134 merchants hawked everything from 20-sided dice to 15-mm.-high infantrymen to a freshly minted Vietnam role-playing game called "Recon").

"I just have this attraction to war," said Kevin Delaney, 12, of Northport, N.Y., explaining why he spent Saturday morning commanding six destroyers in a major sea battle. Kevin's cigar-sized metal destroyers were part of a 12-ship American fleet that did battle with 16 Japanese ships on a patch of Pacific that looked suspiciously like a tennis court.

Precocious destroyer group commander Delaney was sailing under orders from American admiral Earl Wicker, a considerably more adult fellow who works for the Virginia Department of Corrections. Wicker was confident that his superiority in firepower would overcome the advantage the Japanese had in ships and firing range.

The engagement was carefully orchestrated--guided by rules, refereed by umpires, with ship movements carefully plotted using tape measures and calculators and the success of any particular barrage of fire determined by a rolled set of 14 dice and innumerable charts detailing the capabilities of specific, individual ships.

Tim Kehoe, a graduate student in philosophy at Yale, commanded two American cruisers. In the first moments of battle--which took half an hour to sort out as individual ships fired on each other, with ranges measured and damage calculated for each set of salvos--his cruiser "Columbia" took 40 percent damage and started to fall behind its companion cruiser "Omaha." "I'm not gonna last long," sighed Kehoe.

On a bright, hot, flat grassy lawn 100 yards and 200 years from the naval battle on the tennis court, Dave Stephens, 24, of East Orange, N.J., and Paul Silberman, 19, of College Park, Md., prepared to do much more real battle with mace and "bastard sword" respectively. Stephens said he was a 1066-vintage mercenary fighter. He wore a tunic, boots, leather gloves and a metal helmet that bore dents from previous battles.

Silberman wore a black and white tunic--under mail he made himself--and fur leggings over leather moccasins. He was armed with a 40-inch, 6-pound sword whose edges were convincingly nicked.

In the noon sun, Stephens and Silberman casually choreographed a routine, the clanging of sword on mace and shield drawing a small crowd. It was agreed that Stephens would lose his shield and be killed. He died with impressive theatrics.

"It's fun," said Stephens when asked why he spent his free time recreating medieval duels. "It's historical recreation, it's learning what life was like back then."

In the gym behind the field of medieval dueling, miniature model terrains had been set up, complete with buildings, rivers, trees and roads--and peopled with thousands of tiny warriors. On one 28-by-6-foot set the Battle of Waterloo was reenacted.

On a second set of similar proportions, heavily outnumbered NATO forces holed up in a German village trying to beat back three advancing Soviet battalions. The first minute of battle took 45 minutes to play out, as American and Soviet commanders told umpires what they were firing and upon whom, and those umpires consulted charts and dice to see who had done what damage to whom.

The sets, tiny warriors, miniature tanks, and in fact the rules of the engagement all belonged to U.S. Army Maj. Mike Norris, 39. Norris sat at the head of the Waterloo set, Sherlock Holmes pipe in hand, directing the recreation. Norris owns nearly 18,000 tiny warrior figures, which he hand-paints himself. Added to the landscape sets he has built, he says, they bring his collection value to $100,000. But Norris' collection is much more grand than those of most hobbyists.

Norris says he uses war simulation games "to maintain my professional skills . . . One of the things you get out of simulation is an understanding of the cost of war."

"It is a form of preparation," said Norris, who works in the Pentagon, "with a thing that nobody wants to do. But it happens, it's happening now, we've got five wars going on in the world right now."

In the fieldhouse opposite the gym, dealers were set up on two floors offering equipment for every conceivable sort of war game. Tunics were for sale, as were dragon rings, leather pouches, computer programs, board-style fantasy and war games, electronic dice (in the form of a wand called the "Dragon Bone"), one-285th scale M-1 tanks, half-tracks, troop carriers, miniature goblin chieftains, druids, "Roman barbarians" and Union cavalry.

One booth dealt in hand-engraved buttons: "Do not meddle in the affairs of wizards for it makes them soggy and hard to light," "Incorrigible punster--do not incorrige," "Graduate of the Han Solo school of action without thought," "Reality is a crutch for people who can't handle science fiction," "Somewhere just out of sight the the unicorns are gathering."

The button booth also took special orders: "I'd rather be a Dien Bien Phu," "Why don't humans eat their young?"

Richard Magill, 20, of Cleveland, was meandering around the exhibit hall, sunglasses slung in the neck of his T-shirt, which was emblazoned: "Kill 'em all. Let God sort them out." Inquiries about who should be killed were greeted with an ironic smile.