Actually, unless their was some form of treachery, siege was the only real way to attack and capture a town, castle or tower. To get through or over the walls, you needed something, be it ladders, rams, or other siege engines. Simply shooting arrows at towers, especially stone towers, tends to do little to no damage...

Wood arrow vs stone wall... wonder which one stands up better? Sure, you may kill some archers on the walls and in towers, but no way, without ladders, would pikes and swords be able to significantly so any damage to a stone tower before the defenders shot them to pieces.