The portcullis was a formidable contender indeed. First of all, it was very tough on its own. Then, the portcullis was usually set in a thick wall, which meant that there was always the possibility that the defenders could still pelt the invaders with things from above while the invaders stormed through the narrow passage. It was also quite a bad idea to funnel all your army through a narrow passage - that would allow the defenders to simply wait on the other side and kill the invaders one by one as they came through in small numbers.

Thus, portcullises were most often "attacked" by cunning and deceit - an enemy sympathiser could simply open them up and let the enemy in. But that was rare too, as the sympathiser could be discovered and the plan foiled.

In practice, castles were most often besieged. Sometimes the castle owner would plainly reach a peaceful agreement with the invaders, in exchange of some benefits.

Sometimes the siege would go on, until the castle inhabitants were simply left without supplies and again had to open the gates voluntarily, and give up, castle, portcullis and all.

In cases where the defenders were tough or the invaders impatient, the castles were stormed by force. Sometimes that would include machinery such as trebuchets, which would hurl heavy items towards the walls in order to create damage. Othertimes, that would include tunnelling under the castle wall in order to undermine its structure and demolish it.

Battering rams were actually quite inefficient, since they would only work against thinnish doors. Also, the castle builders were quite clever and would position those doors above ground level, accessible only via some wooden stairs, with not enough room to swing a battering ram.

Further read: "Battle castles" by Dan Snow