Extremely, Surprisingly, Considerably
Adverb and な adjective
ずいぶん
随分大胆なことをしたんですね。
You did a very bold thing.
If you manage to make all parts of something work together you worked very, extremely, considerably hard to do it. Doing all of anything is hard so lots of work had to happen.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.