Complex sentence: Windsor was granted royal borough status by Edward I's charter of 1277 which gave no new rights or privileges to Windsor but, as one historian puts it, "recognised (Windsor's) existence and gave it a legal status as a borough".
Split into simpler sentences: Windsor was granted royal borough status by Edward I's charter in 1277. This gave no new rights or privileges to Windsor but, as one historian puts it, "recognised (Windsor's) existence and gave it a legal status as a borough".