Eastern Europe is a very large area, including Czekhia, Poland, Hungary, East Prussia on one side
and Russian principalities on the other side.
Architecture was very different.
In the East (the Orthodox Russian principalities)
most of the buildings were made of wood and I suppose nothing survived. The only exception is churches some of which were built of "white stone". The most famous example is the Church of the Intercesson on the Nerl
This Russian page contains several other examples.
This church is exceptional because it is preserved in its original shape.
It was built in 1158, near Bogolubovo. Bogolubovo was the residence of Andrei Bogolubski,
one of the most powerful princes of Eastern part of Russia. Stone architecture was brought to Russia from the Eastern Roman Empire, together with Christianity. As you see, the architecture
is very different from the Western part of Eastern Europe (Poland, Hungary).
More examples of this period from Russia are found here.
The dividing line between the two parts of eastern Europe essentially coincides with the
religeous dividing line, and roughly coincides with the modern boundary of the European Union. To the east of the boundary you find no fortresses (fortified cities existed but
were built of wood). So the only surviving buildings from that epoch are churches.
For the Western part of Eastern Europe, you can see many buildings on Wikipedia,
for example here