any problem in this sentence

Deleting the a so the sentence reads
The Great Tower was built on land in Belgium
would make more sense.

'land', in this sense is seen as a mass, so we consider it uncountable, ie. no 'a'. To talk about a countable portion of land we use 'piece/plot/acre/hectare/etc.

... a piece/plot/acre/hectare of land

It wasn't mentioned, but of course the past tense of 'build' is 'built'.

Good catch, Ragnel. Missed it completely!

I didn't mention it but,my example did use the word 'built'.

yes i have a bit of a prob with the sentence because

towers are usually built on land especially if we know they are in a country.

Tournai is the only city in Belgium which was once English. King Henry VIII set about rebuilding and improving its defences. A deep moat was dug around the fortress so that the whole district was surrounded by water. The fortress became known as the 'château' (castle) and its keep was known as the “Great” or “Henry VIII” tower.

Good catch for you too, Paddy.

a wee tiny aside; it's not actually past tense. this 'built' is the past participle combining with 'was', which marks the past, to create the passive, 'was built'.