The Imperial Palace grounds are, without doubt, Tokyo's green heart. Located inside a 6.4-km ring of walls and moats that were once the inner defensive perimeter of Edo Castle, this verdant oasis now covers 115 hectares in all, with evergreen woodlands overlooking the moats and creating a very special atmosphere at the hub of the teeming metropolis.

Occupying high ground that once overlooked the Sumida River estuary, the site has been fortified since at least the 12th century, when the Edo clan (whose name means "door" or "inlet") built a fort there. Today, nothing remains of that -- and as a result of fires in the 1860s, and wartime bombs, little is even left of Edo Castle, whose 16-km outer defenses made it the largest in Japan and probably the world when its 1603-51 construction was completed.

Now, though, the palace grounds -- ringed by those massive moat walls, whose masonry was hewn from cliffs along the Izu Peninsula -- has three areas open to the general public. These are the 19.3-hectare Kitanomaru Gardens (located between the Chidorigafuchi and Ushigafuchi moats), the East Gardens (21 hectares) and the extensive Outer Gardens.

Along the banks of the Chidorigafuchi moat, there is a splendid evergreen woodland. Dominant trees include sudajii, also known as shii (Japanese chinquapin; Castanopsis cuspidata), a member of the beech family that is regarded as the king of the warm-temperate forests in western Japan. In May, these trees are covered with conspicuous male catkins; the female flowers are inconspicuous and separate from the male ones.

Meanwhile, the East Gardens contain the few remains of Edo Castle which, from 1603-1867, was the headquarters of the Tokugawa Shogunate. These are the fushimi-yagura (watchtower), fujimi-tamon (defense house) and two wooden guardhouses. The gardens themselves can be roughly divided into two areas.

The ninomaru (inner citadel), positioned on the highest ground, is where the shoguns and their military government officials resided. There also used to be a second citadel on the flat land below the ninomaru, as well as a palace for heirs to the throne. This wooden building burned down in 1867 and was never rebuilt.

Emperor Showa, who reigned from 1926-89, was a biologist and keen nature lover, and he planted a woodland on the site of the old citadel. This now offers a visual reference of what a typical Musashino forest used to look like -- meaning those that once covered the Musashino uplands between the Tama and Arakawa rivers west of Edo/Tokyo.

Here, the tall trees include oaks, maples and hornbeams, with among the oaks both konara (Quercus serrata) and kunugi (Q. acutissima). Before the advent of fossil fuels and chemical fertilizers, oaks and hornbeams were important species in coppiced-managed woodlands, known as zokibayashi, whose timber was used for firewood and charcoal, while the leaves were used to make compost. Yabutsubaki (Japanese camellia; Camellia japonica), asebi (Japanese pieris; Pieris japonica) and the occasional akamatsu (Japanese red pine; Pinus densiflora) are among the few evergreen species found in Musashino woodland.

Right now shaga (Japanese woodland iris; Iris japonica) are blooming in this woodland, and it's also a good time to look out for chigori-yuri (fairy-bell lily; Disporum sessile), a woodland perennial that grows to a height of 30-60 cm and has drooping, white, bell-shaped flowers between April and May. Also growing here are kin-ran (Cephalanthera falcata) and gin-ran (C. erecta), two delightful woodland terrestrial orchids that bloom between April and June, and hotaru-bukuro (clustered bellflower; Campanula glomerata), a perennial with white to mauve bell-shaped flowers.

Moving on, close to the Musashino woodland you will find the suwa-no-chaya teahouse. The original Japanese garden here was laid out by Kobori Enshu (1579-1647), who was one of the most influential garden designers and tea masters in the early Edo Period. Close to the teahouse is a collection of trees from each prefecture in Japan.

The bairin-zaka (plum-tree slope) is one of three paths linking the ninomaru to the inner citadel on the high ground. Ume (apricot trees; Prunus mume) were first planted here in 1478 by Ota Dokan, a samurai who constructed a sizable castle on the site. The large grass lawn of the honmaru (inner citadel) is where the main citadel once stood. Directly beyond the lawn is a small tea plantation of cha-no-ki (Camellia sinensis).

On Jan. 29, 1960, the government made a decision to open the East Gardens to the public, and in May 1963 the magnificent stone walls and gates were designated as special historical relics. Visitors can enter the East Gardens through three gates, one of which, Ote-mon Gate, once served as the main gate to Edo Castle. The massive wooden gate on top of the stone walls was destroyed in the closing days of World War II and was rebuilt between 1965-67. Hirakawa-mon Gate served as the entrance to the third citadel, while Kitahanebashi-mon Gate is located directly behind where the donjon once stood.

Finally, the Imperial Palace's outer garden -- a flat expanse of ground in front of Nijubashi Bridge -- was originally an inlet of Tokyo Bay that was reclaimed in the early 17th century. Now, more than 2,000 kuromatsu (Japanese black pine; Pinus thunbergii) grace the lawn, along with fine specimens of keyaki (saw-leaf zelkova; Zelkova serrata).