When the Normans invaded the British Isles, they introduced an item of Norman law called a "gage of land." Say I was a landowner in need of money; for a sum from a lender, I (the gagor) would give possession but not ownership to the lender (the gagee) until I paid off the loan.
There were two types of gages: living and dead. In the living gage (Norman vifgage) and the dead gage (mortgage). With the living gage, any profit made by the lender while in possession of the land—such as selling the produce from it—went toward reduction of the debt. The dead gage did not reduce the debt, however much the land might have produced for the lender.
During Henry II's reign (1154-1189), he tried to right some wrongs that occurred under his predecessor King Stephen (1135-1154), during which many properties had been improperly seized. In the Assize of Clarendon, Henry created the right of novel disseisin ("new/recent dispossession"), by which the gage could go to the royal court and claim improper dispossession. The cases were dealt with swiftly, which was a plus, but they did not actually determine proper ownership: they merely judged whether the land should go back into the original landowner's possession, and the question of ownership was left for later. Yes, it could get messy.
Novel disseisin made the lender's life difficult, since the gage could at any time make a claim to repossess the land. The practice could easily be abused by a gage.
A frequent use of mortgage after 1095 was for the money needed to afford to go on Crusade. But who had the money to lend? Surprisingly, because of generous gifts, monasteries often had the cash to offer—interest feee, of course. Because so many Crusaders came back with less money than they started with, or no money at all, or never came back because they died on Crusade, monasteries gained lots of land for grazing their sheep or planting vines.
Keep in mind, however, that mortgages were not as common as they are today when everyone wants to own a house. Things were different when generations of families stayed in one building, and cottages could be built by one's own labor, or with the help of friends and family.
Speaking of mortgages and family, however, have you ever heard of mortgaging your children? You will, if you come back tomorrow.