The whole island itself is now a UNESCO World Heritage Site
The New UNESCO site contains churches of extremely diverse architectural styles ranging from traditional northern Russian wooden construction (left), to Byzantine (center), and a late16th-century Russian style with multiple gables and detached bell tower. (Saint Sergius Church 1604)
The beautiful monastery stands upon an island fortress which was the old staging area for Ivan the Terrible's Invasion of the Tatar-Mongolian capital of Kazan. The whole island itself is now a UNESCO World Heritage Site. Check out this video by Russia Insider's youtube channel along with some beautiful pictures below.
The architecture blends Neo-Byzantine (far right in the back) with the national Russian piered construction (center).
Dormition Cathedral (above and below) represents a typical Russian style of church with three distinctive piers. The dome is pear and bud shaped domes (as opposed to the typical onion shape), and gables is reminiscent of Ukrainian Baroque, which inspired similar patterns in Moscow Baroque.
The large church is named in honor of "The Mother of God - the Joy of those who Sorrow". It's architecture is distinctly Byzantine inspired. Notice how it's domes are large and egg shaped compared the smaller Russian onion dome on the left. The church itself is round, cave-like, and enveloping compared to the shaper verticality of the Russian style.
A chronicle mentioning the founding of the fortress by Ivan the Terrible. The cities name "Sviyazhsk" is written in medieval Russian starting on the fourth line from the bottom, the word begins with the "C" immediately after the "oy" and ends with the second letter "O" on the third line.