Medieval taxation was developed specifically in the context of war, and the separation of taxation from war was fraught.

To speak briefly of the development of taxation in England, in Anglo-Saxon times the heregeld was probably first levied in 1012 in order to pay for mercenaries, while danegeld was levied to pay off Viking raders. Under the Anglo-Normans, geld was still assesed on landowners until 1162, while scutage became more frequent. Scutage is payment by knights instead of them providing military service.

At this point tallage was also developed, starting in 1167-8: an aid on movable property and income. This was slowly disassociated from war. For example, in 1168 Henry II levied it to pay for his daughter's wedding.

1215 was in part a demand for consent for taxation: this consent became real (Mitchell argues) between 1217 and 1290. The 1245 and 1253 aids demanded by Henry III for example were refused because they were for the king's personal wars, not for defense of the realm per se.

So the development of taxation actually went hand in hand with war, and 'just war' (such has Richard I's crusades) were one of the best excuses for exacting taxation.

As to how else income was raised: monarchs had demesne lands, which they got all the produce from. The main source of income from their subjects was from the administration of justice. For example in 1245 (the start of a war) the eyre justices were sent to Norfolk, Suffolk, Lincolnshire and Yorkshire, where they usually collected the most in fines.

Edit: formatting and spelling (oops).