greetings derada, welcome hither and congratulations on your wise choice of d'ooge. stick with it son LIVE THE DREAM LIVE THE DREAM.
anyway, piliis longis is of course an ablative, since 'armatus' means armed, thus they are 'armed with long spears'.
certain adjectives do indeed take specific cases, but these will be touched upon (as gratus) in further lessons. besides, most of them can be related to english, as your example 'pleasing TO' therefore dative, and carus, 'dear to' etc. most importantly let your instinct of logic take over when you read and this dispels any ambiguities in practise.