A towered turret, equipped with devices for pouring hot oil on invaders; perhaps a moat, stocked with piranha; or certainly a ha-ha, a deep ditch with a sunken fence -- the National Park Service could have built something really interesting in front of the White House to protect the imperial presidency.

Instead, tomorrow, weather permitting, the Park Service will start digging on a more modest proposal. Its planners hope to install 274 bollards, 38-inch-high, thick cement and aggregate posts, four feet apart, joined by chains, along the 832 feet of Pennsylvania Avenue's sidewalk between East and West Executive avenues. Sandra Alley, a Park Service spokeswoman, said the bollards will be spaced so "a car won't go through them."

Since November 1983, a row of so-called jersey barriers has provided a sort of Maginot line, or Great Wall, in front of the White House. The defensive line is designed to keep anticipated terrorists with truckloads of bombs from crashing through the White House fence.

The Park Service, Alley said, has only enough money -- $465,000 (the contract was awarded to Jones & Wood Inc.) -- to do half the work, from West Executive Avenue to roughly the midpoint of the White House's north portico. It's going to look lopsided, to put it mildly, if more appropriations don't come through by the end of the first phase, to be finished by June. The Park Service seems to be following a method devised by Theodore Roosevelt, who sent the Great White Fleet halfway around the world, leaving Congress to appropriate enough money to get it back.

The bollard design by structural engineer Peter Tomka of Denver is one of those giraffes, designed by committee. The Park Service came up with the design after several years of thought, many prayers to the Commission of Fine Arts (which approves all designs in or visible from federal Washington) and many objections by the Secret Service.

"We struggled with it for a long time," said Charles Atherton, commission secretary. "The security people wanted a barrier that was not subtle, timid or hidden. No one said flatly, but it was obvious that they wanted a barrier that was right out front with it, saying, 'These are the measures we are taking.' The security requirements were to stop some pretty heavy traffic. So the bollards had to be close and heavy. They're not going to have the grace of those wider apart ... in European cities."

Atherton said the commission looked at several proposals, including a stone wall. "That would have been formidable; you wouldn't be able to see the White House. We did the best we could with tapering and shaping the bollards.

"J. Carter Brown {commission chairman} urged the Park Service to put the barriers inside the existing White House fence. But the Secret Service wanted it outside, because they wanted to keep the grounds from being penetrated."

Architect Arthur Cotton Moore, in a 1985 master plan for the Treasury Building grounds, came up with a plaza with a crescent of presidential statues and a moatlike ramp for the east side of the White House. Donald Regan had commissioned the plan when he was treasury secretary and his successor James Baker apparently stuck the plan in the attic and forgot about it.

Moore looked at the bollard plan -- a photograph with the bollards drawn in -- when it was released by the Park Service; the drawing seems to minimize the height of the bollards. And he said, "It looks like a defensive gesture. The White House deserves a plan that's an impressive obstacle, but is still handsome.

"We could have had four- or five-foot-high planters on the sidewalk, curvilinear like those in a French terrace, an embroidery strip of classical shapes. Granite full of earth would be a mighty deterrent to a truck bomber."

The problem of enclosing the White House isn't new. "The White House Gardens" by Frederick L. Kramer, Rachel Mellon and others provides a number of early views.

"The President's house is encircled by a very rude pale, through which a common rustic stile introduced visitors," wrote Thomas Moore in 1804. In 1806, another English visitor said "you may perchance fall into a pit ... The fence around the house is of the meanest sort, a common post and rail enclosure."

Thomas Jefferson ordered architect Benjamin Latrobe to put up a wall, build steps, gates and porters' lodges. The president also planned heavy stone piers set out where Pennsylvania Avenue is now. Unfortunately, his arch of triumph carriage gate, built in the fall of 1806, collapsed one Christmas. Jefferson, like the present Park Service, ran out of money. But he did manage to build the "Jefferson Mounds," rolling rises of dirt, planted with trees that give the South Lawn real privacy.

James Monroe, as president, ordered a stone wall on the north cut down and topped with an iron railing on tall spears, according to William Seale in "The President's House."

In 1966, the Secret Service wanted to put cupolas, revolving armored turrets for heavy ordnance, atop the White House sentry stations.

Only two years or so ago, the Secret Service proposed a plan to close Pennsylvania Avenue entirely to traffic between 15th and 17th streets NW, until an outcry by the Metropolitan Washington Council of Governments made the service, if not forget it, quit talking about it.

What about an ornamental lake, with the White House floating on an island in the middle? The president could stock it with a personal navy.