The castle was quite large, but seems to have been quite susceptible to seige, with a hostile populace outside determined not to let any supply the castle, and with undermining ultimately bringing down some of the walls. Those walls are now replaced with a visitor centre and gift shop (and yes: that is the exit).
We enjoyed the walk around Limerick, and the displays at King John's Castle were excellent, which we followed up with a visit to the Hunt Museum - a must see, if you are ever in the area.